Lithiums are still more expensive on a "per-mAh" basis, but that price is slowly coming down. When we start seeing LiPoly-powered cordless power tools, you'll really see prices start to drop. Many people find the increased duration, reduced weight, and resulting increase in performance well worth the additional cost, though.

Case in point, my Wattage Crazy 8. If not for LiPolys, I probably would've given up on it. With CP1300 NiCds, the thing was a tail-heavy lead sled. Going to 3S2P Etec 1200 packs and a slightly smaller prop saved the plane.