I have also been searching for a set of monitors. I have been doing alot of searching (both new and used) and have actually bought a couple sets just to "see" if they work for me. As you will read through many of the threads on here, the monitors are just one factor along with the room situation. WHat I have found is that monitors will react very differently depending on the room treatment and placement. I bought a pair of Behringer B2031A because they were a good price and they had pretty good reviews. But what I found is that they were horrible for me. Not because they are bad monitors, but because of my room situation and treatment. My situation is that I don't have the ability to change things around alot. So, I pretty much had to find a set of monitors that worked well in my situation.
So what i am saying is, although there can be suggestion about what monitors and such, it really depends on your situaion and the type of music that you are producing.
As far as pricing, to me $500 should be able to get you some good monitors. As for an example... I picked a set of NS10M's for $300 and spent another $200 o the amp. I also picked up a set of KRK V8's for $450 and added the sub for another $200.
Now as far as my findings, THE KRK'S ROCK!!!! These V8's are exactly what I was looking for. Now for the price that you are looking to spend, you could find the RP5, RP6, RP8, or even the older V8's like I did.
As people will tell you, look around, listen tothem, and pick wisely (unless you want to keep changing monitors).
