Just checking it out, I searched on ebay for the R-Pi 0, R-Pi 0W, R-Pi Model B, R-Pi Model B+, R-Pi 2 and R-Pi 3. I did search for the R-Pi Model A and R-Pi Model A+ but there were no Model As and 2 Model A+ at the time of the search.
 
Prices are all over the place, especially with the older units.
 
R-Pi Model A - None found.
R-Pi Model A+ - found only two, both were going for over $45.
 
R-Pi Model B - Here is where the problems are the heaviest. Though some sellers will sell a used R-Pi Model B for around $25, many sellers have them around $40, and it can go as high as $75. $75 for a Model B… Think about it. There is an exception, but he’s not breaking up this lot of 120, which he is selling for $1400. It’s about $12 per R-Pi Model B.
http://www.ebay.com/itm/raspberry-pi-model-b/322410535585
 
He does have another lot of R-Pi Bs that he is selling for $100 for 5 of them. That makes it $20 each.
http://www.ebay.com/itm/raspberry-pi-model-b-5-pcs-batch-/322492099484
 
R-Pi Model B+ - Same problem but not as bad. But the average price for one is around $45.
 
R-Pi 2 – are going for around $40 - $65. It’s a bit high but not as crazy as the Model Bs.
 
R-Pi 3 – are going for around $40 - $55. Again, a bit high but not as crazy as the Model Bs.
 
R-Pi 0 (v1.2 – no camera port) – not found on Ebay. Did find it on Amazon for over $25.
 
R-Pi 0 (v1.3 – w/camera port) – going for $15 to $35.
 
The R-Pi 0’s (w/ or wo/ camera port) are supposed to sell for $5.
 
R-Pi 0W – going for $25 - $50. They are supposed to go for $10.
 
 
Part of the problem is a rumor about the R-Pi 4 coming out soon. This does not fit Raspberry Pi’s pattern of board releases of Sept/Oct and Feb/March. The last board released was the R-Pi 0W and that was in Oct ’15. But Ebon (chairman and technical advisor for the Raspberry Pi Foundation) stated to some PC Magazine that research in the design of the R-Pi 4 has hit several brick walls and its release is delayed until 2019. But then again, he stated that the R-Pi 2 was going to b released in 2017, and it was released in 2/2/15 – two years ahead of schedule. Competition from Banana Pi and Orange Pi forced them to speed up the development of the R-Pi 2 and later the R-Pi 3.
 
So people think that the R-Pi 4 is coming out, it is not. But because of such rumors, prices of the older units are going through the roof. The market can only handle so much. Unlike the R-Pi 0s which were stopped being made and waiting to be sold out, the Model A, A+ and B are still being made in limited numbers because certain projects have a need for the older units, and the Model B+ is still being made because it fits the Astro Pi Project (the unit must fall within a certain operating temperature range and the R-Pi 2, R-Pi 3 and R-Pi 0 run too hot to be used up in the space station). Other projects like Formula Pi also use Model B+ because it is conservative on battery power life. But eventually they will stop making these units and go the R-Pi 0W, R-Pi 3 and R-Pi 4. Notice – these units happen to be wireless units. Is that a coincidence?

I made that statement about the wireless because the R-Pi 2 is being phased out. The excuse is that the R-Pi 2, though it being a Quad Core, it still only 32bits. But Raspbian is still a 32-bit OS. Even the other OSes for the R-Pi are also 32Bit.
 
The problem is the R-Pi 0 and R-Pi B+, which are also 32bit CPU though are single cores. Only the Model B+ can go to space as it does not run hot, a needed option for the Astro Pi. The R-Pi 0, R-Pi 2 and R-Pi 3 all run hot; though the R-Pi 2 runs cooler than the other two and if ran on its minimum MHz, will not heat up.
 
To keep software compatibility backwards and forwards, the OS needs to remain 32bit. This is where the R-Pi 2 beats the R-Pi 3. Running at the same MHz, the R-Pi 2 beats the R-Pi 3. In theory it shouldn't but it does. But the R-Pi 2 does not have wifi or Bluetooth capability. The R-Pi 3 does. So does R-Pi 0W and as the R-Pi 4 will have it. The only reason to keep the OS 32bit is to keep the compatibility between the R-Pi 0W and R-Pi 3; which will keep the software compatible with the older units. There are talks about making the R-Pi 0 into a Quad Core 64bit system or release a whole new board as such.
 
The market however, wants a faster 64bit OS without the headaches of the O-Pi or B-Pi. This is funny, because the 32bit B-Pies and O-Pies are those companies best sellers. Sure they have the 64Bit System, with SATA and 8 or 16 GB of FlashRAM and other goodies. But the 32bit system with the same options are selling better than their 64bit systems. I'm hearing the same being true with the Nano Pi. Just look at Pine64 - a 64BBit board that was to go against the R-Pi with all the bells and whistles; they went out of business in less than a year. Some say it’s the high $75 price. Others say that they were slow in shipping and poor customer support. But the CHIP is $9 ($75 as the PocketCHIP) is a 32bit system that has the same issues as does the Pine64 but is a 32Bt System and it is selling o much they cannot keep up with demand! So you have wait 90 - 120 days, and their customer support sucks the almighty Oscar Meyer Wiener! But people are buying it! And both the CHIP and Pine64 have Wifi and BlueTooth.
 
The major issue here is that the R-Pi was made for the educational market and not for the hobbyists. Outside of the UK, India and China, the R-Pi is the leader in the hobbyist small system boards. In the UK, India and China, it is the leader in the educational market. People are doing things with the R-Pi that were not thought possible, even for such a crippled system (the slow GPU and RAM Limitations it has). There are better boards out there but because of its community support in both the educational and hobbyist sectors, things are being done with it that other small system boards are trying to achieve but has not yet (Kodi for the O-Pi have just been released, is an example, while the R-Pi had it for years. And Mame emulation of the Nintendo 64 is awful on the R-Pi 3 but put it into a B-Pi or O-Pi, it will shine beautifully. This is because of the GPU and not the CPU.)
 
Let see how the market plays out.

You got that right.
 
Just had an argument in one of the prepper groups about "wireless communication" when the SHTF. Of course, there was talk about CB, FRS/GRMS and Ham Radio. But somebody had to stick their head in about "Gotenna."
 
Gotenna is a wireless app and plug-in hardware that goes into your smart phone and claims that you can disconnect your service and use Gotenna in its place for private secure encrypted communications. They claim ranges of 2 to 4 miles with it on a very low power system.
 
It turns out to be BS, as it is a system where it tries to connect to public and open networks and hop into a server like Asterix (PC-Phone Network) and then connects you to your friends and group. Without this digital backbone, Gotenna will not work.
 
But the hype they are selling is that it is not digital and it is "better" than walkie talkies. With my pocket radios, I get 10 miles and do not need a digital backbone.  I also have DMR (Digital Mode Radio) and it can use Wifi or not and have the signal encrypted. I did not mention this yet because there are circle fights as to which equipment is better and some repeater owners are hogging up their equipment for "Friends Only," which is fine. And the range on the DMR radio is the same as the other ham radio I have - about 10 miles.
 
Same with the R-Pies. If one wants wireless on an older R-Pi, a $3 Wifi/Bluetooth dongle to stick onto a USB Port. Problem Solved. Phasing out the older units? Why? Of course, technology must march on. But the software has to match on with the hardware, and that has not happened with the R-Pi. The release of Jessie with Pixel that runs on the R-Pi 3 and R-Pi 0W will run on a R-Pi Model B and possibly on a R-Pi Model A with 256MB of RAM. For the OS, that says a lot. (Jessie Lite will work on the Model A and A+ with 256MB as it does not have a GUI, which is the memory hog; but as for the Model A+ 512MB, it will work on it as it is the same configuration as the R-Pi 0).
 
The CPU of the Raspberry Pi is the same if not similar to the AllWinner CPU; based on the A7, A8, A9, A20 ARM CPU. As stated on the Orange Pi Zero, there are a lot of GPIO associated with these CPUs, and using the same Micro & Machine code. But the GPIO was never furtherized past 40 pin and several of those are either power or ground pins while in theory they can over 96 GPIO Signal pins.
 
Even the IoT demands a minimum of a wired connection as a back up to wireless if it fails. There are pluses and minuses to consider, we both know that. With the right programming, we can have rovers on Mars, as there is no place to plug in up there. But once there is a power and communication node created there to extend range and power, then the option of both wired and wireless will happen like it is here. It is a matter of time.
 
My beef is the price the older R-Pies going through the roof. If I had the $1400, I wold buy that lot of 120 Model Bs and sell half to make up the loss and even make a profit on them (at $30 a piece, it is possible). That would leave me with 60 Model Bs to do with what projects out there I can. The average project out there only uses 3 or 4 GPIO signals, so one is not pushing the envelope as far as I/O is concerned. The Model B fits this niche nicely.