I don't understand this viewpoint.
Barca just bought Luis Suarez for 85 mil, he's 28, a month older than Cavani.
At some point buying top players in their prime for a large fee is very worthwhile for the success it will provide. Even if you can't sell them for much down the road. Barca probably won't get much for Suarez 3-5 years from now, but the success he provides now makes him worth the big transfer fee, regardless of age.
Cavani isn't quite Suarez category, but he's very much worth 40-50 mil, being just turned 28, in his prime for another 2-3 years, with another 2-3 years of likely very good play after that.
If we had the money to sign Cavani, I'd have us spend 45-50 mil in a heartbeat.