I don't at all think GNC vitamins are better. In fact, many times I've made purchases there and found the vitamins or herbs were less effective. I believe they are more expensive because they rent high-dollar properties in malls and pass the cost on to the customer.
I am pretty sure you are paying for the price. Hopefully all of those extra charges are going towards therapy so the owners can learn how to spell rather than just abbreviate. I pretty much despise this store. Year ago I would walk into one and randomly poll the staff to find out what they know and then correct them with proof (love smartphones nowadays).
I don't like GNC. I used to go into the store all the time years ago. They were way overpriced and, like chabella already said, the employees didn't seem to really know anything about the products. What can you expect for minimum wage people, though?