Well that's the thing... you work with a person, letting them choose all of the options, are up front about the costs of the parts all along, and then when all of those $30 parts and $75 bits of art are tallied up they say:
"What, $600?! You said you didn't need a huge profit!"
Sure, you can build a MAME cab out of $150 in cost and parts lying around. But those parts lying around are yours and not stuff to just be given away. Those people wouldn't just give you $350 in DVDs because you wanted to watch some movies.
It really comes down to, you can pay for the parts, you can use spare parts lying around, you can spend tons of time doing things the long inexpensive way... but either way it's going to be roughly the same base cost. You're either paying in inventory, time, or money. It's never "I just took this $50 bill, a cab I found in the trash, and in an hour had a functional decent looking MAME cab."