Since the AR-15 platform is very customizable, you have to decide what is your end need for the rifle. Do you want a varmint/target gun, M4, pistol caliber carbine or a regular m-16 style rifle? Try AR15.com to find more info about different models, and ways to customize your rifle. Then try your local dealers to find out their prices on Bushmasters, Rock Rivers, DPMS and Olympic Arms with the options you want. You can also build your own rifle, which is easy to do and slighty cheaper than a store bought one since you don't have to pay a Federal tax on the rifle.
PS: If you happen to live in a state that has its own Assualt Weapons ban, you will be limited on the military features you can put on the rifle. For example No bayonet mounts, 20-30rd magazines, claspable stocks, flash hider and etc.