My giant female burms cost me less to feed than my 6 to 8 foot raiser females that are growing much more rapidly. The reason for this is simple: Burmese that are young and growing can require a lot more food/energy than burmese that have all but stopped growing and require just a moderate, bi-monthly meal under a maintenance diet. I cite this irony just to demonstrate how it can actually be cheaper when they are larger.
To answer your question outright though, it depends entirely on the cost of food for each individual herper and what is available to them in their area. Here where I live rabbits are commonly 5 or 6 bucks for a 10 to 12 pound rabbit. But I have read posts from other herpers that can't find comparably sized rabbits for cheaper than 12 to 15 bucks each. A large burm, in my opinion, really doesn't need more than 2 rabbits a month of this size. The energy that they require to live is far less than they will derive from 20 to 24 pounds of rabbits per month. Plus, this still allows for growth without gross excess calories that can lead to obesity. So I would say that the range in terms of cost would fall between 10 and 30 dollars per month per large python. I myself raise my own rabbits and it is closer to about 3 or 4 dollars per month per python.
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David Beauchemin
High End Herps.Inc
http://HighEndHerps.com