There is a point signing him if, and it is a BIG if, he manages to show he is still a world class striker in the remainder of the season. I'd also argue that at 28, there are some years in him yet. RVP was signed for 22.5 million at the age of 30 and although he is in decline, it was largely due to his form that we won the title, the season before last. Don't think many would argue that he was a bad signing.
However, Van Gaal is right. He needs to prove himself. He's a wild card loan signing. Can he prove his fitness? Can he prove he is the world class striker he was before his injury. Can he prove he can adapt to the Premier League? If the answer is yes to all, then 30M may not be too excessive. At 40 million, personally I'd look elsewhere.
I do feel for him (as much as you can feel for someone earning circa 300K/week) as
1) He's not a tosser
2) He's working his ass off, for the team
3) We aren't exactly creating a shed load of goal scoring chances. 3-5-2 !!!!!!
4) I think he needs an extended run in the side, to get his mojo back... He hasn't had that.
Crucially, I find it difficult to see how he will succeed here, given that the team just isn't playing well and for that reason, I'd wager he'll be shipped out.