80% AR-15 Lower Reciever

One option is to buy an 80% lower receiver.

There is some work in getting it to the point where it will be a functioning firearm, but one main advantage to it, beside building it yourself, it the fact that you can have it mailed straight to your house.  No need for FFL transfer fees.

A mill would of great assistance, but I have seen people use nothing more than a dremel tool to finish the remaining 20%.  There are drilling jigs that would provide for more accuracy in some of the operation.

The next step would be finish.  There are instructions on the web on how to anodize the lower at home.  You could also apply some type of spray or bake on finish.  It really just depends on what type of finish you are looking for and what the firearm will be used for.

You can usually find 80% lowers for less than $50.00, some dealers offer multiple discounts also.