Its a douche bag move to overcharge for regular six packs. Ive done what the OP described multiple times. A store by where I used to live (a Sunoco gas station of all places), had really great prices on Mix Sixers (with a good selection too). But their prices on regular six packs were on the higher end. It was ofter cheaper to do a make your own 6 pack with all the same beers. I dont see a problem with it. The employees certainly never seemed to care.