I'll help a little
Firstly I would look at the Trijicon ACOG. Whatever anyone says, just have a look at some on their site:
http://www.trijicon.com/na_en/products/product1.php?id=ACOG
See what you think and then look on the EE. There are a few good prices on some, one of which I think just popped up today as a bump. Think about what you want to use it for at the range. The ACOG is a sight which is kind of multipurpose. I mean that it is very handy at various ranges and has been used by various military forces for the fact that its robust, reliable and accurate. But you pay for that....like REALLY pay for it.
Plinking? If so, Maybe the ACOG isn't worth dropping that much cash. A option instead would be something like a Millett DMS scope 1-4. A LOT cheaper and does pretty much the same thing.
But I suggest Don't go crazy though when buying your first AR scope. I mentioned the ACOG because of your budget, but you may turn up day one at the range with it setup on your rifle, and want more magnification or maybe you want to have a better eye relief.
If I were you though, I would suggest just getting a aim point scope AND a medium range scope like a vortex or something. You have the best of both worlds then
If you want longer range, swap out the aim point and whip on your medium power. For Closer ranges and more rapid acquisition then pop your aimpoint on. Whatever you choose make sure your happy with your decision before you buy. I have made the mistake of buying a scope people told me was great to use with a AR. Found out it just wasn't what I wanted it for
You just need to figure out what you want from your rifle and then match the scope to that
Whatever you choose my friend, welcome to the AR scene and happy shooting