I'm in the Army now but used to work at AAFES in Gordon (Augusta). Gas is purchased from same supplier most of the other local gas stations get their fuel from.
The price is cheaper at AAFES due to a few reasons: AAFES sets the priced based on the average local rate minus a few cents, and they don't make as much of a profit on gas. At Gordon they are making their money off of the stuff the Basic and AIT students buy....and liquor/beer sales...who else sells on Sundays?
But yeah 93 octane on post is more than likely the same as 93 off post with the exception of some places like Chevron. Oh yeah they used to sell 100 Octane in North Augusta, next to the Toyota Dealership on the Aiken-Augusta Hwy.