Transfer fees are not set in isolation, so comparing them is always tricky... he probably would have been cheaper if he wasn't coming out of the Premier League, or if he didn't have recently signed a new long-term contract, that sort of thing.
Just a few years ago, I'd have expected a player such as Sigurdsson, moving from Swansea to Everton, to have cost somewhere around 15m, and van Dijk maybe somewhere around 25m.
Things have changed a lot recently, Sigurdsson moved for 40m and van Dijk 75m.
Ultimately it will all come down to how good he is for us, and what he wins for the club.
United once paid 30m for Rio Ferdinand, back in the days when 30m was a ridiculous sum of money. If you adjust for transfers inflation, Rio was probably more expensive to United then, than van Dijk is now for us. But nowadays no one even bothers to mention how much he cost, because he proved to be an extremely influential player in a very successful side.