firstly - how old is the cow, is she purebred AA or AA cross? How old is the calf and how far pregnant is she? (ie how long is a piece of string?)
Assuming, she's purebred (with no long pedigree), up to about 5 yrs old with @6 month old calf and @ 3 months pregnant, I would say £900+. Buying the whole family is always considerably cheaper than buying individuals. In a market they would be cheaper and also less because it's winter. I breed Herefords and have a pure bred unregistered heifer with 6 month old bull calf and her due again in 6 months. If I put her in Selby market at this time of year, I would be lucky to get £800 for the outfit.