Barcelona won't offer Gundogan huge wages, and they wouldn't have to as he'll be delighted they would see him as a fit for their team and the successor to Xavi, doubt they'll offer Dortmund big money either. Perez won't offer him insane wages at Real Madrid, either, as he's not a Galactico or done anything to date that sets them off on their wankfest campaigns. Gondugan isn't the kind of midfielder they fawn over. He'd be paid a decent wage at both clubs, but nothing mind-boggling like what is being suggested. Incentive and performance driven contract with a strong tilt upward as reward is how they'll play it because they have no need to break the bank given so many players gladly sell themselves short to go there and prove themselves.
It's the sugar daddy clubs that will pay obscene amounts to land him precisely because they have to.
If he ends up at Barcelona, it won't be because of money or the best (or anywhere close to it) financial package, same goes for Real Madrid to a lesser extent. I'd be quite certain we would offer more in wages than either of those clubs but we don't have anything else to turn his head, unless he's a fan of what we've done in the past and has an interest in playing for Manchester United.
If the sugar daddy clubs didn't involve themselves, I reckon in pure financial terms, we would offer the best package for Dortmund and the player.