Gosh, we don't buy milk by the gallon! In the UK, we are much more into moderation and restraint! ;D Our big cartons of milk are 4 pints, which is half a gallon. 4 pints costs £1.18 at the cheapest, so a gallon would cost £2.36 ($367). However, sometimes there is a deal where you can get two 4-pinters for £2 ($3.11). People who are lactose intolerant and use soy milk have to pay a lot more of course.
Weird that the price varies throughout America. Do you not have chain supermarkets that are in all the states, and which have set prices? Or can the same supermarket charge different prices depending on the state? The prices I cited are from the websites of Tesco and Asda and Sainsbury's - those supermarkets are throughout the UK and charge the same everywhere.