Someone out there will probably suggest you talk to me. I can design one, and plan to, but not until next year. The interface hardware is easy, but DMX-512 isn't one of the pre-assigned USB functions. That means I'll have to write, or hire a software guru to write, PC drivers for it. I've never even tried to write a Windoze API, so I've got a steep learning curve ahead of me and several other products to get out first. Until I get mine out, I don't know if there is one available anywhere for much under $500.
Ive used a home job one which was made by a company that makes electrical gear which was relativly cheap, about 350US but Im not sure they are arround anymore cuz i tried to get a balun box off em but couldnt find the company anymore