Not too long ago AR's were plentiful. New AR's from reputable company's sold from $800 on up, Pre-Ban's commanded a premium and generally went for $1100 and up. If you were handy and resourceful you could build one up yourself and end up spending as little as $600.
Fast forward to TODAY. In the present world AR parts are very difficult to come by, and when you find them sellers generally want a premium. Complete rifles that were once $800 - $900 are selling for MUCH MORE than that now. Pre-Bans are commanding BIG premiums over what they were a year ago.
Who knows if these market conditions will continue? Prices certainly could continue to go up, perhaps even drastically if new laws are introduced restricting them further. Then again, the market could settle down and prices and availability could ease.
All that said, it takes a lot of effort to wear one out. Don't get me wrong, every part has a "useful life" then again almost every part is replaceable. If you are buying just make sure you get an accurate assessment of the condition of the rifle, there are plenty of folks here that could help with that. Bottom line, if your lower receiver is in great shape, there is nothing else that can not be fixed or upgraded if you have the time, the skills, and the money to do so.
Colt's have always seemed to command a premium over most other brands, personally I never saw a reason to pay it. There are MANY quality manufacturers out there, I would leave myself open to many of them.
Good luck on your search!