I think he's earning his money. He has to buy the rgb board, chassis, db23 connector, switches and whatever else he puts inside the box and I'm sure it takes him a couple of hours to put one together and test it. Not to mention the time he spent figuring out how to do it and make it work. If you buy the raw board, you have to figure out how to make it work with an Amiga in a stable configuration. Then, if one gets damaged in shipping, he has to take it back and either fix or replace it and probably has to swallow the shipping charge.
I've thought about trying one of these Arcade converters but I've got too many projects going on right now. I know I'd spend some time putting it into a chassis if I got one.