It's a good question. I'm not an expert but based on numbers I've seen I reckon at least £50m a year gain for City. Barca say he makes them 20-30% of their revenue which is about £615m according to Deloitte. So he brings in £120m - £180m. His contract is 96m a year (confirmed by Barca) so that's consistent. If we're conservative and say it's roughly a 66/33 split in his favour that's about £50m.
For comparison
£70m - win champions league.
£150m - win premier league
£250m - City's current Commercial revenue.
So you're talking about having the best player in the world on the pitch + roughly the equivalent of the European Cup prize money every year.