IIRC, Ronaldo took a very minimal pay cut in gross wages to come here, and Messi makes a hell of a lot more.
Why would Messi take more of a pay cut to come here? Because that is what it would take. You would be asking him to take a greater paycut than Ronaldo did to come on board.
Messi has a contract until 2021, so only one more year. I think his net wages are around 40M, which for barca is around 80M gross. For us the same would be around 50M gross I think (or less). We (and inter for that matter) would have a headstart from the get go, thanks to the tax law in Italy.
Now, we don't know as of now who he is in talks with and what his demands are. But I pretty much doubt anyone will pay him 80M a year (maybe with the exception of PSG and City, but they're in FFP trouble as it is, do they need more smoke?) on a multi year contract. Let's say he settles for 20M a year (and the remaining 20M he was to make during the last year on his barcelona contract, we pay him through a sign on fee), that's less than 100M for the total cost of the whole operation. That's what we paid for Higgy WITHOUT his wages. For a guy that had 20+ goals and 20+ assists in the season that just finished. Do you guys think that would be a bad deal?