Personally, I dont have much money to dump into bikes. Locally, there are no hills so I'm one of few people that actually invest in a mountain bike. A few of us, however, have noticed that BMX bikes don't have the tenacious edge to handle our cheap Florida soil. I have a cheap bike... a Mongoose XR200.. a cheap walmart fab. It cost me a slim $157.99 and was actually worth it. It was either that or a schwinn, but I wasn't too keen on the feel of the ride.
I know most people laugh at the walmart fabs, but... cheap and useful.
Remember, a BMX doesnt have a 21st speed.