emotionally devastating | how many cans of formula do you go through?

my son is 3 months old and has been breastfed. my supply used to be great; not sure what happened. he's been super fussy lately and gnawing on his hand so I offered him a bottle after I breastfed him for 25 minutes while he squirmed. well, he downed 8 ounces. 😰
now I feel like I've been starving my baby so I'm 80% sure I'm switching to formula. I will not have him go hungry. 
how many cans do you eff moms go through a month? what size? we are trying to ballpark what to budget for formula. thanks!