He's on the last year of his contract, just like how we bought Vidal for 10m and like how RM got Ozil for 15m etc.
He was still the equal top scorer for Man City last season with Aguero. And not to mention the 'issues', known to hate Manchester and he said he wanted to leave the club, which forced City to sell probably for an even lower price.
You should read up on these things more

rather than going off what you know from 8 years ago