Taken in isolation (and regardless of the player’s relative merits compared to other players, which is always subjective) then yes, it seems like a lot of money.63m is an awful lot of money to spend on a player who isn't as good as Mousa Dembele.
However in a generally rising market, 63m for this 22 year old may be looked back upon as a snip in 5 years’ time. Sissoko is a classic case of this - it seemed ridiculous two years ago that we paid what we did, but now - it seems like reasonably good value.
Whether you agree with it or not (the money has to come from somewhere .... our pockets) it is the reality of modern football. You have to be prepared to take risk in order to advance.