There are so many talented players who simply do not realize their potential. Signing one of them is a bit tricky as you can never tell how they would turn out. You'd also have to consider how much playing time a player who is not a star and just has potential will get at a club like Manchester United.
Take Lindelof for example. He wasn't always starting games here last season and when he made a mistake he was slammed from pillar to post by both media and fans. It's a big club with big expectations and loads of pressure comes with it. Not the easiest place to establish yourself.
Maguire didn't have the perfect season. He made mistakes at times that would have seen him benched at Manchester United but he didn't have such obstacles at Leicester. He was able to take the opportunity to play regularly in the league and develop himself so that he made it into the England setup and, of course, he went on to perform well in the World Cup.
From Manchester United's perspective, we would be signing a player who is ready to start for us now and is ready to hit the ground running, which may not have been the case a season back. Sure, we are spending more, but we are now signing a player who will definitely start for us in my view compared to signing a player with potential who may or may not work out here. And that is worth the price in this crazy market.