Pato would play instead Beckham, not Borriello. Pato played more as a winger this season. Him and Ronaldinho on wings were really deadly combination....I'm not so worried by the Pato loss as I am for the potential loss of Nesta for this game. Borriello has actually been playing very well so even if Pato was fit I think Marco would start.
As for his price, well Galliani has said he is as important to Milan as Kaka' and rates his talents as good as Messi - so is not for sale (when Chelsea were trying to buy him last Summer). But the very fact that he has compared him to Kaka' would suggest he has a price, Kaka' went for near 60M but he has a marketability + Golden ball and CL medal pedigree. But Milan did pay a lot for Pato considering how young he was and was playing in Brasil, so would want to see a good return on that investment. I would guess 40 - 45M.
They lost to Inter b/c 1) Abate's horrid mistake to put them behind early. 2) Beckham offers absolutely nothing besides great crosses. Sometimes the crosses lead to goals, but when the other team knows that's the only thing he does its not hard to anticipate the cross and only send 1 player to mark him. 3) Dinho was marked well and didn't have a great game.
Pato, along with Dinho/Borriello upfront would strengthen the team immensely. Beckham isn't a RW, he could work however as a RM with Pirlo and a holding mid.