Given the price of second-hand hardware (Ebay), then I believe that the $300 for a new 040 or 060 accelerator would be very competitive, assuming it is stable.
As to numbers... who knows? I think 50 is a given, but more than 100 ...?
Some people would buy it partly to support the effort, partly out of curiosity, whereas others would buy it out of a genuine desire/need for an 040/060 accelerator.
If you do manage to get a working prototype and - eventually - a production run, then A1200 accelerator would definitely be the way to go to get the numbers. Personally, I'd like to see an A4000 version aswell though.
Regards
Rich