Still, you can see how various factors have forced the price up (Man Utd's desperation, January window, buying from a rival) because I really can't see how you can say Mata is worth more than £30m (and that's me being somewhat generous).
I'd say various factors forced the price down, lack of playing time and his injury, because when a player has combined 50 goals and assists in the previous two years, in addition to being a key figure, protagonist, if goals and assists don't already indicate that, in my books is well worth that sum.
Yes, his price was significantly higher last summer. If Chelsea wanted to sell him back then, they could have asked at least 40m. When a player is on the sidelines for too long, or doesn't get enough opportunity to consistently show his talent, his price will undoubtedly plummet.
if he's not for sale or they can't replace him, then by definition Chelsea's valuation will be greater than the valuation of his other highest bidder, it's circumstantial. for instance, Chelsea have already played ManUtd twice this season, Mata can't harm Chelsea in the league anymore but he can take games from ManCity or Arsenal or Liverpool. So Mata can still have value for Chelsea while he playes for ManUtd this season..
Yes, there is a lot of money in football nowadays. Imo many clubs can pay that much money, if Chelsea had to rebuff offers, they would value him at something like that.