Baby formula costs are killing me.?

My son goes through a large can of formula in less than 4 days. It costs me $28 for each can. The costs are killing me. I don't have a lot of money and it worries me. I try to feed him more baby food so that maybe he will drink less formula, but he isn't too keen on the food yet. he is 6 months old. How old do they have to be to have just regular organic milk? cause it would be so much less expensive. but i don't think that he is old enough. i applied for wic, but they said i will have to wait ATLEAST 3 more months cause the huge waiting list.