I get the food from Amazon, it's not that expensive and I don't have to carry it that way. I usually get fatty mixed grains, sun flower seeds and peanuts. In big bags (like 20kg each) the prices are reasonable, about $2 per kg in average. For the breeding season I may add some insects, which cost a bit more.
In general, they can only eat as much as you give them - and they find other stuff elsewhere, especially in summer. They did live before you started feeding them, remember. :)