I don't think it's directly money related because Milner knows Liverpool wont pay him as much as we pay him now, let alone what he wants out of us for his next contract.
I reckon these new incentivised, bonus related contracts we are now giving out is stopping him signing a new deal to a certain extent because you have to be playing games to get the bonuses to kick in and therefore get a nice wage, being a squad player in the past few seasons he may see himself getting a lower contract offer than he is currently on because we might not be able to guarantee him game time and he needs to be playing to get the big wage.
I reckon he thinks he has won a fair amount at City and now fancies a position of playing regularly in a new surrounding with a different challenge.