What is the difference in quality btwn the $47-$99 mountain bikes and the $200 - $2,000 bikes?

We are thinking of buying a bike for my 13 yr old daughter for Christmas. There are lots of specials (this weekend, WalMart has a Mountain bike for $47.77). I've also seen bikes priced from $200-$2,000 at places like Sports Chalet. She is very athletic (a cross country & track runner) and would use the bike for fun and for some cross training exercise. Would appreciate feedback on how far we need to dig into the budget on this one...Thanks!