In summary
17s were new and rare, so they asked 5 to 10 k over msrp
18s hit the market that was almost filled, and so folks got 1k under msrp
19s hit a market thats full, so they go close to what a normal f150 sells for.
20s will hit a market with Sale Sale and Sale signs everywhere
Isn't the 20 supposed to have more new features? I am just hoping my 18 survives long enough to get there!