I've never really done a cost evaluation on mine. And the number of tortoises I own tends to fluctuate.
I just bought a $25, 25-lb bag of Mazuri tortoise diet. I give them a cup every 2-3 days. This is only supplement for their regular diet of fresh produce of collard greens, various lettuce, yellow squash, zuchini, as well as the occasional apple, banana or other fruit. I only have 2 sub-adult tortoises at this time. I would "guess" mere $10 a month...ish.
Its hard, because sometimes I get fresh veggies for free from friends