Japan has been sucked dry over the past 5 years. Forget Yahoo Auctions, eBay is the best bet.
You're not going to find anything cheap unless you wait a long time and snipe an auction.
Not many people use the white PC engine. It's RF only and most consoles have yellowed by now. Possible to fix but ugly to present as a gift.
But the big thing is, why hasnt your friend just bought one by now? As a poster above said, the coregrafx complete is about $70, little more than a new PS4 game.
You'll also need to gift a game or two to go with that console. Probably your own to save money.
But honestly if the timing isn't great, just don't bother. Christmas already passed and you gotta put your own family first. Save every dollar.