275/40x17 BFG G Force sport, $145 ea on Tire Rack. Even the BFG KDW2 in that size is only $183. Sure Michelin and high end Goodyears clip the $300 range, but in general, V and Z rated 17" tires can be found a whole lot cheaper than V and Z rated 15" tires.

When you get a chance, read through the link. There are even more links within it pointing to those providers of decent 15" tires, like a 295/40VR15 Michelin http://www.summitracing.com/parts/COK-71231/