Gotze had a stupidly low release clause considering, most world class players don't have those unfortunately. It would perhaps be more silly to use his example as a yardstick as that of one of Madrids buys.
As for your second point, the obsession with numerical output is clouding your judgement. Di Marias contribution to goals at Madrid is stupidly high, his ability to carry the ball up the pitch, and break defenses down purely by dribbling at them is something they're going to massively miss when he goes and they know it. You're also letting the likes of Ronaldo and Messi cloud your expectations there too. Di Maria got 17 assists in the league last year - the second highest in La Liga was 13, the highest in the Premier League was 13.
Di Maria would bring us much needed pace. He'd add quality and natural width when out wide, and a terrier and an actual ball carrier in the middle which are two things we massively lack - no other player in the world could bring both to us. The tactical flexibility he would offer us is huge since he can bring qualities we lack all over the pitch pretty much. A midfield 3 of him, Herrera and Carrick would be beautifully balanced and a midfield worth getting excited about. Having him running down the wing would bring us the classic gung-ho wing play that we associate with United.
David Silva got 7 goals and 9 assists last year. That's hardly a great numerical output is it? But would they have come close to winning the league without him? When he's not playing, their control of games suffers hugely, their creativity in the final third, they play infront of the opposition and offer little threat in comparison when he is off the pitch.
In comparison to Suarez moving for a similar fee. Remember, he went for less because of the behavior, the ban and that Barcelona seemingly had no competition as Suarez was willing to push for a move there. Di Maria hasn't got those issues and we have competition for his signature so he's in higher demand.
A player is only worth what someone is willing to pay for him. If you look at the players Barcelona already had, they needed Suarez far less than we need Di Maria. Whether that be because we haven't got pace, because we're short in the middle and on the wing where he would bring quality, because we're in need of a confidence boost right now etc.etc.