Denilson was once also the most expensive player of the world
Rossi is just not as loud as Sanchez, that's only because of Sanchez' great technique which makes him hot on the market.
By no means does it mean that Sanchez can offer more. It's not the first nor the last one that you see how a player with such characteristics is becoming hyped. Another example is Neymar. Who in his right mind should be ready to splash 40m for a player like him? That's crazy. It just doesn't mean that he's worth 20m (or any) more than Rossi who's got the advantage of being already proven in Europe top leagues.
I'm yet to watch Neymar, so I can't really comment on that.
As for the part of your post I put in bold, I can do nothing but agree with you. It could, easily, be proven right.
Here is the catch: Sanchez is extra talented. Teams see a potential world class, a player who can be one of the very best 3 players in the near future. Someone who'll be worth 60-70m eur. Then again, he can simply flop and be sold for 10m euro. With Rossi you more or less know what you'll get. Not a super talent. He'll never be the very best. But he's a very good player who'll most probably always do well and be worth ~25-30 million euro.
So it's the potential that attracts teams to Sanchez, even though the future might prove them dead wrong.
But with Aguero it's a different story. The boy scores +20 goals four seasons in a row. You know what you get with him too. But he's got enormous talent with which Rossi wasn't blessed and you know that he can reach the 60-70m eur price tag one day. Even if he doesn't and he stays what he is now, he can still always be sold for 30-35m eur.
That, and not their origin, is the reason for their price tags.