I'm not questioning that he is in a better bargaining position, I'm questioning how much of a difference it makes. Telling all the other players on £150k that the reason Reus is on £250k is that he cost them very little in the grand scheme of things won't stop them from wanting a pay rise.
You're looking at it purely as wages. Take Pandev as an example, he had a dispute with Lazio and had his contract terminated and signed for Inter on a free - he received a signing on fee of anything from €7-10m. He was a £10-15m player, he's not getting that sort of fee had Inter had to pay Lazio to sign him.
Do you really think Mendes won't be telling Real and whoever else that he/De Gea wants their cut of however much Real save? Whether it's wages or any other fees, the less Real pay on De Gea (in this example) the more De Gea will end up pocketing.