Nobody is going to be paying 400k, not even us. When DDG himself doesn't warrant it, a 19 year old definitely doesn't. He won't be getting that, more like 250k-ish probably, which even he came out and talked about (when dismissing rumours). More could be some add-ons for performance bonuses.
What will clinch is the Raiola fee, which probably Barca baulked at. That, and a first team undisputed start which is also a factor.
Also, people seem to be forgetting, whatever the rumours, de Ligt is not going to make the move till August. Due to Raiola's ban. Too many things can change till then, and ****, nothing has been confirmed. All of this could still be Raiola stretching Barca to the last mile to get a deal done. Neither United nor Barca have come out and said anything. In fact, latest rumours out of both camps have been suggesting looking at alternatives for de Ligt.
But in all honesty, if it is 400k, I would be against it. It will become untenable in a couple of years time.