I don't know the exact formula but the value can change a fair bit. For example, I had Darren Bent at the beginning of the season and I think he started out at about 7 million before topping out at about 9.5 after people picked him left and right. I've seen some of the better teams have values of 108-113 million at various points in the season. In addition, last year you had to pay a sell-on fee so if you bought someone for 8 and he was worth 9, you only got 8.8 (I think it was a 20% fee anyway).
Not sure if that answers your question but it's all I got.