This is the highest rated player in his age range in the world and the hottest gettable player out there. The last one was Fenkie De Jong, who is now on £300k+ at Barcelona - even in 2017, Dembele went to Barca on £200k after just a season at Dortmund, which I believe was only basic.
You mentioned players in another stratosphere that (at buyable ages) would cost far more in transfer and wage than Sancho.
Sancho will go for the going rate of an elite, world class attacker who is not even in his 20's yet. It's not even a case of being overpayed; his agent would be incompetent not to get him a package in that ballpark as that is what players who have done what he has should be getting and going for.
The perception of Sancho has to change from young promising talent etc. to acknowledgement he is performing like the best players in the world, and has a claim to be the best RWF on the planet behind Messi. He's definitely in the conversation alongside Mbappe, at the very least.
As I've said a few times in here, the potential for him to cost more than touted is there if he has a storming run in the CL and Euros, which is essentially how Bale got himself from one bracket to another in the blink of an eye - you mentioned Madrid/Barca getting away with paying others less, which isn't accurate (Bale £600k; Saurez £300k; Griezmann £300k(?) etc.).