you can get a pretty decent bike for $400. Anything less than $400 is probably going to compromise quality so that you'll have to upgrade parts in a short time. I have been getting into mountain bikes for the last few years and BMX is a LOT cheaper than other kinds of cycling.
also check Austin Craigslist for a decent custom complete bike. BMX is huge in Austin and some people might sell their $1200 custom complete for $400.