If WoW lowered their prices to Jagex's, it would kill both games. RS would die because people would quit en masse to play WoW, and as a result, WoW's community would become flooded with all the derpy 'tards from RS.
This made me laugh, the WoW "community" (if you can even call it that) is diabolical - in no way is it better than the RS community (but then again it definately isn't worse). Plus WoW is free to play now (for the first 20 levels)...
It wouldn't be WoW that kills RS. It will be Guild Wars 2. The game looks amazing.
GW2 is only one, Diablo III and SWTOR, and to a certain extent some lesser known ones such as ArcheAge and TSW. Runescape wont die though at least for another 5 years - the player base is still very large and people play because it's cheap, easy to access and easy to play. But then again, great games such as Age of Conan and Lord of the Rings Online are free and have great gameplay.