One calf should only need one bag of milk replacer. I would recommend the medicated milk replacer which in our area of PA is $40. We begin weaning our calves when they are six-weeks-old. When they reach six-weeks of age, we feed them half the amount of milk replacer each feeding instead of going cold turkey. They should also be receiving calf feed of some kind. Our calves have feed placed in front of them as soon as they are a couple of days old. They can have up to 2 1/2 pounds of feed twice a day. If you are raising this calf for beef, it should have free choice fresh grass or hay.
I would recommend the medicated milk replacer which in our area of PA is $40
Sounds like awfully cheap milk replacer to me. You should be getting an all milk, milk replacer-NO SOY. Remember in milk replacer you get what you pay for. $80 is what we pay for a 50# bag and it's worth it. I also know that is about right across the country for a bag of good milk replacer.
I do agree to slowly thin out the milk replacer as they get older. Usually, as they get eating more and more grain their manure will get looser and looser and that is a sign of when to reduce the milk replacer. (It is the protein amount)