It's fairly obvious:
1) Cavani is going to be wanted by many clubs, but realistically only a few can afford him.
2) City and Chelsea are two clear candidates, but Real Madrid could be interested too... plus, it's not entirely impossible that a couple of other clubs might be lurking in the background.
3) Cavani fancies Spain. He's in no rush to choose now, as he and Napoli hold all the cards (so far).
4) It's now a game of poker... Napoli will want to hold out for the highest offer and hope a bidding war ensues. Their worst case scenario is that one or two of the clubs in the running pull out and buy their 'plan B' striker instead, in which case, Napoli just sell the the remaining bidder still at a very high 55M (give or take). Napoli can't really lose here.
The only safe way for City to handle this (safe in terms of not being hung out to dry) is by offering (say) 60M (or 40M + Dzeko) with a take it or leave it NOW ultimatum. If they don't take it - we move on. Otherwise, we'll be strung out all summer, and could end up not getting him anyway, with no options left (since plan B strikers have gone).
Of course, this means paying a little over the odds, but that then gives Napoli a decision to make... "twist or fold?"... once we pull out, they might not get a 60M offer thereafter, and other clubs will know City are out of the running.
The same goes for the player and his agent. 'Here's the deal, it's exceptional'... take it or leave it. Be honest with him... "it's a superb deal, you have 48 hrs to think about it. If you need more time than that, then you're not the player for us anyway".
He looks a fantastic player. But he's not Ronaldo. He's still a gamble (albeit only a slight one IMO).