Peoples perception of transfer values is incredibly skewed after the Mata buy.
Chelsea took £37.5m 1. because we wanted him that badly to have a marquee signing 2. They were selling to a domestic rival. If he was going to somewhere like Madrid or Barca he probably would have gone for around £28-30m.
50-60m for Reus is laughable for many reasons. The most of which is Dortmund arent sitting on a pile of cash you know, they arent a financial super-power we just have to dangle quadruple his wages and hell be chomping at the bit to join. Offer a respectable 35m and I think we can land him, his agent company would be getting a sizable slice of income over the contract period, their stars will be playing for the biggest commercial team in world football which will likely lead to lucrative deals with Nike or Addidas for player sponsorship. The Ronaldo comparisons will spark up "can Reus be as important as Ronnie was" then theres the "new look United".
Would Dortmund sell? of course they would, do you honestly think they would turn down 35m that they can buy a replacement for Gotze, Reus and Lewandowski, you have to remember they are currently rebuilding and dont have the financial muscle to just "buy" they first need to sell.