I don't think they have gallons, they do have milk, I believe the sizes to be about a quart. As for the price you are at a resort, resort prices are in effect. I never look at the price so I can not help you there.
We've bought milk there although I don't remember the price. I suspect we looked, winced and bought it. The last time I looked at a price there - $3.75 for a 2-liter bottle of Coke, I wanted to cry. I rarely pay over $.99 at home. Buying off-site brings considerable savings, if you can work it into your plans.