If he played to his potential, sure he'd be worth 70 mill. As it is...
He has during the first half of both his season, and even at that he's worth around 40 million, maybe 45. People get transfer numbers skewed because of teams like PSG etc. Teams like Chelsea Psg etc pay 40-50 million for players because they can. The teams they buy from demand big money because they know they can get it. Prices have been seriously inflated over the years.
Anyway there is no way Fabregas is worth £70. Madrid paid £80 million for Ronaldo. Not only is her far superior to Fabregas but he also brings Huge commercial benefits $$$$. Fabregas doesn't have that start factor to make money for the club commerically. 80'000 fans didn't turn out to greet Fabregas at the Camp Nou.