I assume here that you are asking about nearfield active monitors, and that you are aiming for sub-$500 for the pair (not each).
There are a few factors which play into the decision, including (a) the type of music you will be working with, (b) your workstation setup, and (c) the room.
The type of music can determine whether or not you need a subwoofer. If you do, that's an additional component which you will need to factor into your overall budget.
The workstation setup will play into the positioning and power of the speakers. How much space do you have? Where will the speakers go, and will they be near any walls or corners? How will they be positioned relative to your head? (assuming nearfield)
The room can play a bit into the monitor decision, but it's probably a bit more of a factor in tuning your setup once you have your monitors. Look into room acoustics as you're making your monitor decision to understand what your overall monitoring/room setup will entail.
Having thought a bit about the above, the key thing for monitors is to have something sufficiently detailed and revealing to make accurate mixes. Ideally, it won't be too fatiguing, as you will most likely be listening to them for extended periods of time. On top of that, it's mostly a matter of getting accustomed to them so you can make accurate mixing decisions.
The net of all of this is that monitor choices are fairly personal. Keep in mind that the goal is to get something accurate which will allow you to hear everything you need to hear, which is NOT the same as "sounding good". Take some CDs and audition your top candidates against music you know well, focusing on key portions that highlight particular ranges or potential problem areas (extended low bass, detailed highs, stereo imaging, etc.) It can help to take some of your own mixes, if possible.
Some specific inexpensive monitors I've heard good things about are the KRK Rokits and the lower-end Events. Again, the key is to be familiar with the speakers so you can make accurate mixes.