Last September at the BW General Store it was $16 for a six pack and the prices are normally the same at all WDW stores. Buy off property if you can for normal prices.
A 6 pack would indeed be about $20. If you're only getting one six pack may not be worth the extra trip to near by gas station. If just a few could be worth it. If for more than 12 beers, garden grocer is definitely your friend.
Fairly standard except there is a big difference between a bud light and a more craft beer. Also an individual bud light will run you about $6, but in a 6 pack it's cheaper.