Under $500: I found the Zeiss Conquest line to be unbeatable. Then the Bushnell Elite Series. Under $1000: most ACOG's. Then Leupolds. Under $2000: Schmidt & Bender.
Could not buy one new, as they are astronomical in price. Searched for a used one and found used for $600 recently. Scour the internet and use this site: http://www.riverofguns.com It's like Google for Guns and Accessories!
Just stumbled upon this great resource of AR 308-info! Currently I own only one (DPMS AP-4 with Quadrail) 308 AR platform, but always looking to upgrade. AD military here but close to retirement.