AR-15's are substantially more expensive than a mini-14, if you buy the whole rifle. If you are willing and able to do some work yourself and have some time to wait, you can save a lot by building one yourself. I built my first for $540 and I could have done it for cheaper. Check out the AR15.com board. They have detailed instructions on how to build one and the discussion forums have a lot of info as well. There are a number of sources for kits. Getting a lower receiver is the only really difficult task as they are in short supply these days. If you are interested, I'll post some possible sources. Cost for a lower can run $90 (what I paid) up to $150. It took me about 2 hours to assemble and the rifle shoots great.
------------------
Dorsai
Personal weapons are what raised mankind out of the mud, and the rifle is the queen of personal
weapons. The possession of a good rifle, as well as the skill to use it well, truly makes a man the
monarch of all he surveys.
-- Jeff Cooper, The Art of the Rifle