On the one hand I´d value him just above Ronaldo despite his dominance at Barcelona, because that quality has only been shown in one league, country and team, wich makes it a bit more of a gamble.
On the other hand he is so important for Barcelonas dominance in Europe. For teams like Real and Man U, the actual value is probably an added £100+.
Realisticaly though, if you tried to get him right know, I think anything below 400 wont cut it. But nothing is priceless and there is probably a point where he himself feels he has to leave as well, to do the right thing to the club.
Knowing Barcelonas history they might just sell him for 65.
Barcelona: "We want 400"
City: "60"
Barcelona: "390"
City: "60"
Barclona: "380, final offer"
City: "65"
Barcelona: "Ok thats fair"