For just one, it wouldn't be much at all after you got everything it needs. That can cost upwards of $100 depending what type of set up you are looking to get. Bedding is free if you want to go the newspaper route, which is what I do

For me personally, if I bought rats rather than bred them it would cost me about $200 a month. But, since I breed rats and keep my snakes in a rack system, I think it costs me about $40 a month, and that's mostly rat supplies. So even with a bunch of snakes, its not much once you get past the intial cost of getting all the supplies like tank/tub/heat/thermostat/etc.