$850 is a bargain for a Jersey cow and a heifer calf IMO. You could probably sell the heifer calf, bred, for that next year. I don't see how you could go wrong.
1 1/2 gallons of milk a day while feeding a calf is a good milker for a Jerseys - depending on diet and other things. I don't think she is a poor milker. I think I would buy her and see what you can do. I bought a pair about like this last year for $1200, used the cow for a nurse cow, bred them both this spring and sold them this year. The heifer sold for $700, the cow went for $1000 after using her for a nurse cow for 7 months. Probably one of the better livestock choices I have made in my long history of poor ones :lol: Actually made money on that deal :lol2: