To set up a tank, that's with heat and everything else, would run you around $100 to $150, depending on how much you spend on decor, etc. That is also for a new 20L tank with everything new. A tote with heat and everything would probably run you about $50 to $70. For one snake, ordering mice, unless you live really close to a supplier and can managed not to pay shipping, isn't really worth it. To buy enough to make it less than most pet stores, you'd have to buy more than your snake would eat in a year. At Petco prices for food, you are probably looking at $6 or so for a baby to young adult snake and about $10 a month for an adult snake. It's actually a bit cheaper or about the same price a month to feed 3 or 5 snakes, buying bulk mice than 1 snake at most local pet store prices.
Most UTHs use around 15 watts max and with a dimmer or thermostat that you'll need to keep the right temps, they use less than that. It barely costs anything to run one or several.