RoadRash is dead on: I loved my first bike, I learned more about working on a bike because of that bike than I have on any other bike since then. I would NEVER recommend a bike over about 300 bucks to anyone who is getting into the sport and I only recommend entry level bikes designed by serious bmx companies to beginners so they are at least on a somewhat SAFE bike.
But, when you start spending more cash you really end up paying into more brand name parts and stronger stuff. A newbie who doesn't know how to maintain a bike to begin with is going to destroy a $500.00 bike (mechanically) just as fast as they will as $200.00 bike. Stuff falls apart, and most beginners don't have a clue how to properly maintain and adjust their bike.
That's not even including the thousands of kids who buy bikes to ride serious bmx, then quit after 6 months or a year. What a waste a $500.00 bike is on those kids... or more accurately, their parents.
The DK Cincinnatti remains one of the best valued entry level bikes on the market for street/dirt/vert. The DK Signal blows away (completely!) any other entry level flatland bike out there. For racing... ask someone else, I'm clueless.