I, too, live in the US. I pay about $17 for 50 liters of the Carefresh bedding, and I go through that amount in give or take two months. Because my bun uses her litter tray most of the time, I don't have to totally clean her entire cage each week, which is the main reason for the bag lasting for that amount of time.
I looked online and was only able to find 10 liter bags for those of you in the UK. Leave it to us Americans to ridiculously oversize everything...