You could write an absolute crap 200pg book for $600 that no one will buy, or you can write a decent, 40-50 page book for $600 that people will buy. The choice is really yours. There's something to be said about getting what you paid for, and with freelance writing, that's certainly the case. If you pay a crappy wage, you'll most likely get crap in return. You simply have to pay for quality.