No shit, sherlock. You act as that money gets out of your pocket. The very best quality is worth it. Buffon, Thuram and Nedved were worth it. They were top3/top5 in their positions before they joined and they keep being the best at Juventus for many years. Mind you, Nedved was "getting old" with 29years at the time.
This would be better applied to Van Persie than Cavani, but Cavani is still undoubtfuly an amazing striker.
Marrone, Giandonato and Appelt. Is that enough to you?
Youngsters should come from the youth sector and not for shitloads of money in most of the cases.