Pato is on approx 60k per week at AC Milan, I'm sure a large number of Liverpool players are on greater than 60K per week, so the salary would not be that large in the scheme of things, if it was a swap deal.
The only concern I would have is with Pato's injury problems, however if you told me at the start of the summer we would sign Borini, Dempsey and Pato I would be a very happy bunny.