Zoo Keeper sets are a real pain to get and keep running in my experience. I spent about a week on and off getting that set up and running and testing and had it fail twice during that time. Yes, I think $475 is a lot of money for a working boardset, but I've sold them in past for $350 (about two years ago) and I have had at least 10 phone calls and emails for people looking for them, so I figured I'd start high and see what happens.
On those DK boards, again I think $250 is a lot of money for a board, but we've sold quite a bit of them in past six months (at least 6 of them) and my stock is running low and people keeping paying the asking price. I think some of our collectors are not do it yourselfers and rather pay more knowing what they're getting is guaranteed to work and that we stand behind it. We do ship a lot of boards out of the US to collectors who appear to have deep pockets.
I will have another set of Zoo Keeper boards for sale shortly, I just need to bench test them for a while longer.
Qix sets are similar in some regards. The video processor board is exactly the same and the sound board can be modified to run Zoo Keeper. The main rom board is the same but requires different jumpers for the larger Zoo Keeper roms. Most Qix boards I find and Zoo Keeper boards I find need a bit of work to get running, they are just so touchy. At some point I might start offering some repair services on these as I'm getting better at fixing them, but I still have a few sets to go through here.
Anyway, sorry to babble on, just wanted to shed some light the on pricing, it basically boils down to what people are willing to pay and this is how I make a living so if people want to pay more for the product I'm not going to say no. I do try to give back to the community with goodies and technical information when I can.
Anthony