It depends on several things.
Caliber
Brand
Material
If you want a long lasting, decent one. Plan on spending around 1,000$
If you want a cheap, .22 caliber one, Mossberg sells them around 300-400$
$800-$2,000.
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
Colt has not offered a AR15 in 308
$1599
Try AR15.com Try AR15.com
Both are assembly line weapons. Stag in my opinion also has a better reputation. Unless you decide to go tactical, they are all pretty much the same. Bushmaster, dpms, stag ext..
If it is one of the recent Colt .22LR AR15 rifles, then the answer is no - they are completely different internally.
As of today (10/18/08) $900 ~ $1,370 depending on condition.
1959
Yes
By it's serial number. Go to www.proofhouse.com and under 'long rifles' listing look for AR15 Sporter
@ 40 inches.