Well, if you start cow's milk too early and your son has one of many subtle reactions that typically go undiagnosed (increase in colds and ear infections, rashes and dry skin patches cropping up on face or body, stool problems like loose stools or constipation, etc.), you might end up "saving money" on one end but LOSING money on the other end (doctor's visits, stress and exhaustion of parenting a child who is sick and allergic).
There is a darn good reason why babies are supposed to get breastmilk or formula for a MINIMUM of 12 months. Cow's milk is best for baby cows - NOT optimal for the growth and development of baby humans. Please find other places to cut back in your budget other than your son's formula. He *needs* it, no matter what any other mom says on here about how her kid was "just fine starting on cow's milk at 8 months."
I really am *not* trying to be harsh here -- I know you are doing the best you can and that you're trying to take wonderful care of your child. I just want to be honest with you that cow's milk is not the best choice. If you're short on formula, ask around to see if any breastfeeding moms you know are getting free formula from the companies that they'd be willing to give you. Check on eBay for sellers with 100% ratings that are selling unopened formula that is NOT expired - you can often get a deal that way. Or switch to generic formula instead of name brand. There are ways to cut costs WITHOUT switching to straight cow's milk.
Signs of cow's milk allergy, in case you start him early on cow's milk and want to keep a close watch on his health to see if he is *OR* is not tolerating the cow's milk:
http://www.allergysa.org/milk.htm
http://www.drpaul.com/illnesses/milkallergy.html
Best of luck!