Considering that, it costs a hell of a lot more then $15 per year per snake. a more realistic number would be at least $80 - $100 a year.
$15 is a bit too optimistic. But I think its probably more in the $60 range. I spend $1 a week for small rats, so figure $50 a year on food, plus a few bucks on bedding and the electricity to run the heat and its in the $60 range per animal.