He shouldn't cost much at all. Might even make us money. When there was a sniff of a deal back in 2013 we'd lined up increased agreements with Etihad & Nike, plus had new deals with other parties ready to go. They paid his fee and basic wages.
A lot of his earnings come from the commercial deals he has, both personally and through Barca. Rooney has that arrangement at United, where he would front up an advertising campaign for a sponsor and the income would be split between the club and him. United weren't paying him £300k+ a week out of their own pocket.
Messi's salary from Barcelona is not 300k a week though.
It's €104,000,000 a year.
€71,000,000 a year wages, €33,000,000 in image rights.
That's before the €70,000,000 loyalty bonus paid out over a 5 year contract, and up to €20,000,000 per year in bonuses based on trophies and ballon d'or wins.
If we won everything and he won the Ballon d'Or...it's getting close to €140m.
And that's his contract from 2018. He'd probably want more to move away from Barcelona.
This doesn't mean he doesn't pay for himself, but we would need to add probably over £100m a year in commercial deals to cover it. Our current commercial revenue is...£230m all in.
I'm pretty conflicted on it. On a footballing level I'm not sure it makes sense. Where does he play? Will he be worth it all at 33, he hasn't been great this season by his standards. Part of me thinks he should retire at Barcelona. It would annoy me that Pep would be forever branded incapable of winning the CL without Messi if we win it with him.
However...it would transform the club. Commercials would be boosted forever, name and brand recognition would skyrocket, global fanbase would grow exponentially.
Also...we'd get to watch probably the best player of all time play for us every week.
And think of the things he could teach Foden, Sterling etc. Players will throw themselves at you to play with him.
I think it would be a no-brainer for the club but I'd feel a bit odd about it until the minute he pulled on a City shirt.