Depends on how big them rats are. It isn't too bad for medium rats (150g) but bargain for anything bigger.
Why don't you breed some yourself. They aren't difficult to breed. I guess feed and upkeep can be fairly high maintenance but if you only cull what you need at a time, the rest continue to grow and your snake would certainly enjoy a nice, warm, fresh feed. I probably only need 2 boys, 6 girls to keep you in feed for a couple of snakes. Cheap plastic tubs from Go-Lo with holes cut into lids and covered with gauze to prevent escapees, or replace the entire lid with wire mesh lids then you can feed and water them without having to lift lids.