The biggest difference in string quality is not in the materials used. It is the construction method. Some are much more careful and use techniques that make for a better string (better servings, no creep). You are not paying for string, you are paying for knowledge and technique.

Buying from a large company with great technique (like Winner's Choice), will cost more, but the string will preform very well. There are some great values out there, including some individual string makers , like Berry Strings, that will give you a great product at a lower price. I make my own, and they are the best, simply because I am so darn meticulous and careful when making the string for my own bow.