they get a comission on anything sold, but the GNC products will for instance cost them $3 for box of vitamins, and they'll sell it for $30.
so while a good box of vitamins would cost you $50, if its not GNC, GNC had to pay $30 for them.. so instead of a $27 profit they'll get $20.. just an example and not real figures, but you get the idea.
a girl i know works at GNC and youd be amazed at what GNC products cost them.. almost nothing. Im pretty sure the vitamins had the largest markup.. it was something like $1 for a huge bottle
most of the people i know goto GNC once a week to buy their "old" products, bags of protein for $5.. etc they have a rack of goods they have to sell for nothing or throw out.. only place to shop there