It will be significantly cheaper to breed in the long run if dollar amount is all you care about. (Other 'costs' include smell, space in your home, time, etc.)

So what you are really asking is, how much does it cost to set up a breeding group?

Buy mice, a container, bedding, water bottle, and food. That is your over-head, and for the next 20 years of the snakes life you just have to have food for your mice. Have you already searched the forums for this topic, I am sure it has been discussed in depth in the past.