You need to go sign up for
www.ar15.com
They will have the answers for you - bottom line? It depends if you try to get one stripped down or will you get the bug and start adding rails, lights, ACOG sights etc etc... It CAN be cheaper (I think you can save a tax if you buy the upper and lower at different times), but there are some really inexpensive basic ar-15s out there.
The cheapest I've seen (I'm thinking about getting one actually) is the Olympic Arms 'Plinker'
http://www.olyarms.com/?rootView=bro...;ids=plinker-s
also, Impactguns has a very wide selection of AR-based guns - I use it to windowshop and get ideas:
http://www.impactguns.com/store/ar15.html
Hope that helps =)
PS - they make ARs in other calibers as well, FYI