i have no opinion on de ligt's current market value. i haven't seen him regularly since he left juve. you might be right with that 50m evaluation
just one remark: the offer might be influenced by his wages. he's on 16m gross (13-14m in gbp) which would be the 3rd highest base salary in the current united squad. and that's also part of a player package. they can pay 50m for a youngster no problem because he won't and can't demand +10m figures, but if a defender costs 30m per season (including wages, amortization, agent fees, bonus), that might be a bit too much even for a pl team