Lets check... look at the cost of Toilet Paper, cold remedies, spices, canned veggies and fruit, baking items, .... stuff like that. If you use coupons, like everyone else here, you may not have to pay the sales tax at the commissary, like you may at the regular grocery store. But you may have to pay the "surcharge" at the commissary.
I live an hour outside from post/base and I save quite a bit.. about 100 bucks every trip to the commissary. But I pay 30-50% more than you do. You probably pay less for milk and such. Just keep in mind how long you will be there and compare pricing around you.