From what I understand the difference between finishing 4th and 2nd is £4m, look at how we played against Spurs it takes van Gaal a while to get going, this is normal, it is no coincidence he hasn't performed that great in champions league group stages before, if De Gea turns out to be the difference between getting knocked out in the group stages or making it to the quarter final that's another £9m.
I'm not saying De Gea will win that much money for us but going on the previous two seasons and a new team that's integrating it is possible he could be such a big difference, getting knocked out of the group stages or failing to mount a title challenge has future consequences of its own, not just the less prize money but also it means we might have to pay even more for the best talent as the weakest english champions league team or even struggle to attract players who would rather go to clubs where they are sure they will challenge for trophies, if we can sign the best talent then that helps us commercially.
A lot of factors go into this, if the finance team have calculated we would be much better off challenging for trophies than not (but still getting champions league), I can see why they say no to £20m - £25m, especially when you consider had De Gea signed a new contract we would pay him £200k per week, that's what we value him at, we have a £200k per week player this season for £75k per week, that's £6.5m less in salary we are paying him.
I can see why the club are saying no to a fee under £30m. The Moyes season cost us a lot of money (Deloitte estimated £50m for not being in the champions league alone), we want to sign another world class player but none seem available, it would be unwise to let one of our few quality players go especially after Angel di Maria left.