If Chelsea can get 50m for Luiz you can sure as hell get 60m at least for Sanchez
It doesn't work that way.
Firstly,
There are only two possible destinations for Alexis and that is London or Turin. He don't want to go to Man Utd(No CL) and neither do he want to join Liverpool(Personal reasons, he prefers London in England). PSG and Man City are not interested and cannot get him anyway due to the FFP sanctions imposed on them.
So you either demand 50m and risk losing the 40m Arsenal offered and Juve can't afford him anyway, atleast not at anywhere near that price. Real Madrid got more for Mesut Ozil since they would have been fine with having him at the club either way and PSG were just way too stupid enough to pay that much for David Luiz.
The only way we can get 50m is if we are fine with keeping Alexis at the club if we dont get what we want and if the buying club are desperate or see that certain player as their utmost top priority signing.