Steve, here's my take on the math....
I realize this is an acoustic guitar forom, but I'm using electric guitar strings in my example ( how many acoustic players change strings every month? And of those, how many have time to go to GC every month?).
GC string club member: $20 to get in the club, you walk out with one set of strings. You make sure you go back every month, and end up with 12 packs.
Non-club member, Looking to beat GC's "deal".... walks into Best Buy, drops $16 on the counter, and walks out with a 10 pack of strings.
http://m.bestbuy.com/m/e/product/det...=1218108416268
If he really wants the full dozen, he does end up spending a total of $23.60, buying two single packs along with the 10 pack.
But, our BB shopper doesn't need to worry about missing a monthly appt at GC. Miss one month, the GC club member has spent more on strings, along wirh gas, and time.