You can get pre-packaged sandwiches in the US (and a bunch of other stuff too, including hot food). I don't buy them and don't know the exact price, but they aren't expensive, definitely under $5. Eating from corner stores and gas stations is just usually looked down on.
Pre-packaged sandwiches in the US are typically more in the $8-10 range, and that's just for the (sad) sandwich. It also tends to be much more depressing than what you'd get in the UK. Barely edible.
