You'd be surprised at what people actually pay for these days. I seen stuff on ebay both custom and official that were obviously made with lesser quality materials than that Custom Goku, sell for just as much if not more than what this guy's asking. (Highest I seen SS Vegeta go for was $499, for crying out loud!)$200 is reasonable for this custom piece (although still expensive, it should still leave some room for profit for him). $250 would be the highest I would go, though. Also consider this IS a one-of-a-kind and any other customizer taking commissions would ask for at least $200 anyway, and I don't blame them. And the chances of an official Figuarts SSj4 Goku being released are extremely slim, at the rate they're releasing figures, mmmaybe we'll see a prototype 2-4 years from now. And even then they could just as easily put him in the Figuarts Zero line.
I wish he had a "Make An Offer" option and I'll throw him that no problem.