Hmm it really depends on what you're looking for - with good monitors that are flat (as in they don't have a boost in any range, eg in bass, making you reduce the amount of bass in your mix in those speakers and ending up with one that really wouldn't have enough in others) I'd suggest you fork out a little bit more, maybe within the $500 range.
The KRK Rokit 5 have good responses, and go for around that amount. Others you could look at are the Yamaha MSP3s or the Behringher Truths.