Breastfeeding to formula

Hey guys. So my baby is 7 months and has been breast feeding her whole life. It’s been hard to even get her to eat baby food more than once a day because she always wants boobs so I’m iffy on even bothering with formula. Now I’m leaning towards starting her on formula but want to build up at least 3 months worth of formula because there’s times where I don’t have money and don’t want her to go without if she does like formula. But those of you that do give your baby formula, about how many cans of formula do you go through a month? And which one is better? Powder or liquid? This is what I have so far