Well I think as with vertonghen it's about getting in reasonably young and early. They are about to be found if you have the correct scouting personnel.
We had a fairly clear run at Jan from Ajax for £12m (Ditto Eriksen for 14m). FFWD 5 years once PSG fucked up the market up; De Ligt costs Juve £67m (+£10m agent fees).... Not looking to be as good as Jan either.
A lot's changed since then........ As we're learning; unless you get these guys super-raw, by the time they start to consolidate their talent and gain a little experience, everybody wants them and their price-tag goes through the roof.