http://www.retrocollect.com/News/hdmi-out-modification-gcvideo-comes-to-the-nintendo-gamecube.html
Don't know if it's available yet, but I've been keeping my eyes on it.
If you're so inclined and have an FPGA programmer, OSHPark has PCBs for it.
The project is open source, so you just get the files off the sourceforge page.
I did a bill of materials once, and it would cost about $90 to make one, without any external packaging. The majority of the costs is the PCBs, which you need to buy in sets of 3.
If you did a set of 3, it would be $60 each.
All this assumes you'd solder it and program it yourself.
If one were good at finding a manufacturer that would fab the PCBs and assemble/test them on site, and did an order of, say, 100 units, the per unit costs would likely be around $45-50.
You'd still need to program them, though.