What possible reason do we have to actually want Cesc? Just because he grew up here and now wants to come back should have absolutely nothing to do with a long term financial decision that will set Barca back a few quid.
Basically, the way I see it, Fabregas has signed a contract extension that will put Arsenal in the box seat to get the most amount of money for him. Also, the fact that our players/management have not exactly been subtle about wanting Cesc is another thing in Arsenals favor. Finally, most reports claim that Arsenal want Cash PLUS Thiago, which is a major no-no IMO.
Thiago has stuck it out with the team, did not leave to get better chances and tried his best to break into the first team when given the chances, that is the type of commitment you want from someone who is going to replace your midfield lynchpin.
Finally, the difference in quality between Thiago and Cesc is noticeable at the moment, however, with more games in him Thiago will get a better understanding with our first team members and he will get much better.
No need to spend 30M+ lose a very promising player for someone who has not even been that impressive this season.