"Jason is right about 15" being cheaper than 17" tires. I just looked.
15" Tires
17" Tires"
The Goodyear MTR/Kevlar is a good example and literally $100 per tire difference, at least per the above link from Discount Tire Direct. So if you're buying a new set of tires (I always go with 5), you'll expect to pay ~$500 more to dress your 17" wheels versus your 15" wheels. Crazy. That's why if I ever buy a JKU I'm buying a set of 15x8s in a 5 on 5.5 pattern and would still come out ahead that first tire change...
Now, if only someone could tell us why the price hike for the 17s...