Baby cows grow well on cow's milk.
Baby humans grow best on food that is designed specifically for them -- preferably their own mother's milk.
If that's not available, then donor milk from another mom. And if that's not available, then infant formula. Straight cow's milk is not included in that list of infant foods, and for good reason!
If your doctor is truly looking out for your child's best interest, they will urge you delay cow's milk for as long as possible -- the earliest would be 12 months, but waiting longer is smart too.
Yes, formula is expensive. But the high price of formula is NOT a good reason to switch a baby over to cow's milk earlier than recommended, regardless of the reason the child is on formula (many moms *choose* formula over their own free breastmilk, without even really trying much at all to breastfeed, and then complain about the price of formula! For other moms, breastfeeding didn't work because of not having a knowledgable pediatrician or doctor to help them get off to a good start, or because of a true medical reason, etc., and formula is a lifesaver).
Stick to formula for now, and scrimp in other areas of your life - that's what I'd do if I were in your shoes.