I'm afraid you don't get anything decent for that money unless you check out the secondhand market.
I bought a (=1) new Behringer B212A monitor for my vocals a couple of days ago. It gets the job done, but there's nothing extra about it. Only that one costed me 280 euros, equalling about 400 dollars....
You need a mic ($50-100), a small mixer ($50-100), two speakers and an amp, unless you go for active speakers. And of course cables ($50). You're going to have a hard time finding all this for less than 500 bucks, me thinks.
Perhaps, if you're really short of money, try plugging the mic straight into a used active speaker. I just don't think you want to do it this way for too long...