Once you get your initial setup done (shouldn't cost you more than a few hundred at MOST), the only real cost of keeping a snake is food (I pay $0.63 a week -Canadian- for hoppers), and the occasional substate change (again, depends on what you chose to use, and how often you change it).
That's it. I'd say $4-10 a month. IF that. Food, and substrate. I pay $0.63/week... I use forest bed substrate ($6 new) and have never changed it, but I do bake it once and a while (every couple months).