yea 20's have gotten quite cheap over the years. IMO the pirelli tires they put on stock f150's are worthless, but some guys love them. overall diameter and load range really sets the price on 20 inch and bigger tires, and the fact that most lifted applications are going to a 20 inch tires, helps drive up the price, but just like 18's were bloody expensive a few years back, the 35x12.50-18s are getting very cheap IMO compared to years back. it just takes times...

 

Think of it this way - your tires have an inside diameter - the diameter of the hole inside the tire (ID) and an outside diameter - the diameter of the outside of your tire (OD).

If you increase your ID or rim size (same thing) but you leave your OD the same you actually wont be affecting the performance of your truck or require any more clearance to make it work. It's when you go to a taller tire with a larger OD that you need to worry about lifts, levelling kits and speedo adjustments...