There ARE good bikes for $300 to $400. I highly recommend Specialized Hardrock XC.

check out website:

Hey Chris-

For 300 - 400 I think you’ll get a low quality bike that you might not be so happy with. I do believe for that money you can get a used ride that someone has built-up, but had upgraded from. In Seattle with all the possibilities there you have to have a good ride.

Places to look: Pawn Shops, Friends that have been riding for a while, bike shops that do consignment sales, Craigslist and e-bay. Make sure you protect yourself by test-riding before plopping your money down. I have a friend that bought a MTB off Craigslist - bike looked great, but there was a flat spot in the rim and those are incurable.

Good luck, MTBing is way fun!