What John and Steve are saying is very true, if you keep an eye on ebay they are out there, not all the time but they do come up. I got a Cromemco 21 slot the other month for $40, granted it needed some work.. but they do exist. if you can get ahold of a board that isnt in bad shape you are off to a good start. however for those of us outside of the US and who dont want to go through redirect companies for shipping and end up spending 2-3x the item cost on just shipping, getting a bare box/ chassis or even a board is a little bit more difficult. if one were to want to build a brand new board then yes.. its gana cost.. as we see above.
with respect to the ATX power supply to drive a system, what Steve is saying is true, they dont have huge caps in them, they can deliver current and are tried and tested for all of our computers todate, but the +-16 and +8 of a sizable system, you would really want to test it out or move to a big transformer bridge rectifiers and big caps. the problem we end up meeting again is cost.. transformers and big caps are expensive.
the best bet IMHO would be to hold tight keep your hat on and play the waiting game for a while, saving some cash at the same time. if you get lucky on ebay with an old system/chassis and/or board (thats in good condition).. for a fair price then your onto a winner. if you have done enough waiting and there still nothing around that you like.. you weigh up the options of new gear.
with respect to the mentioned 18 slot board, i think there seems to be a little confusion about how the ATX PSU and board tie into the backplane. please donot confuse the two. the backplane itself (no atx board attached) is a separate board from the ATX board. the kit is 2 (two) boards. one 18-slot backplane board and one ATX PSU board. the backplane board is a normal nothing special 18-slot active terminated S-100 bus board, the Active termination is a direct copy of the circuit CompuPro use for termination and the same termination used on John Monahans board supplied by the computer project (so i take no credit for this). it will wire straight up to your +16, -16, +8 and GND power inputs of your S-100 compliant power supply, you can use it without the ATX board. For intents and purposes you can throw the ATX board at the cat and forget about it. the backplane board is just another active terminated S-100 bus board. the ATX PSU board however is a separate board that has a 24 pin ATX connector on it (accepting the 20 or 24 pin ATX plug) it has a terminal block output that you 'could' wire up to the input of your S-100 board backplane IF you 1. did not have the need for exactly +16 -16 and +8V. 2. wanted to wire up +5 onto your +8v rail if you ran cards without onboard voltage regulators, 3. could wire up the +12,V -12V, +12V to the "+16, -16, +8v" rails if you wanted. you could even (by way of modification wire up 3.3v to any other rail on the s100 bus if you needed 3.3v out somewhere). the ATX PSU board is just a nice little board to have if you have a small system (or large ) and dont need the exact IEEE696-1983 voltages on the rails. and have vetted your ATX PSU for output current and "quickness to supply". +12V input to a LM7805 will still give you +5 out. Or have cards without voltage regs, you choose what you put on your power rails. It also works prity nice to use your salvaged ATX supply for a lab PSU. the ATX PSU (we could call it a sub-card) has: +12v, +5v, +3.3v, GND & -12v output on its terminal block. The ATX board uses a small PIC microcontroller to act as the "debounce" switch (on/off) as its about expensive as the amount of 7400 series devices i would have used to put on the board to do the same job. i realize this may cause some "Ahhhhh Errrrrr" for some users.. but the code and programmed devices can be supplied if needed. I just thought i needed to clarify this before any pre-orders happen or before i even post pics of the board design.. Note: these boards are a "work in progress" i still have to clean up a few tracks & footprints. please excuse the long message.. especially Vince if you have not been caffeinated yet ;-)