Marketing strategy,brand recognition and operating cost, when a company is paying big money for a top player to endorse their string they will jack the price up, when a company has brand recognition they will jack the price up, when a company has a large over head of operating costs they will jack the price up.
Other than that there is little difference, Babolat and Luxilon charge a large amount for most of their polys, because they can get away with it, if it costs more and a top player is using it people will automatically assume it is better whether it really is or not, than they will use it and hype it to others to justify it in their own minds.
There are not that many manufacturers in the world most of these strings are produced side by side with just a subtle change, some don't change a thing but the price and packaging, yet people will swear they play differently.