We actually agree. What I am saying is we have huge needs at no. 9, arguably GK. That would entail what 100-120m£ (esp if we spend 50m on Costa)
But we still need a CM to support Eriksen, a RB and a better 3rd choice/partner to Martinez. That's where I am talking abt being smarter and buying 20-30m talents.
That's why I have no issues with 10m for Pellistri or 20 for Amad.
The big difference, particularly with Amad, is that he had done nothing to justify his fee. At 18 he had played a handful of first team games, and scored one goal. Endrick has achieved more at the age of 16, as has Moukoko at 17. Money has to be spent wisely, even on teenagers, meaning that the player should have a) demonstrated his ability to perform at a high level, and b) fit United's style of play. For example, there is no point spending big on a centre back who can't play a high line, or a full back that is useless going forward, etc. - you get the idea.
In the past few years Real Madrid have done a great job of buying youngsters that look like world beaters (Vini, Camavinga, Tchouameni, etc. and tried to get Mbappe and Bellingham). This is a good strategy, but they are targeting the top talent, not some random names who look great at under 21 level. This approach also works better when there is a stable, successful squad to help them integrate, rather than them coming in as an automatic starter and with the expectation of being the saviour of the team. Two of our best ever players were bought as expensive teenagers (Ronaldo v1 and Rooney).
Under the radar signings can also work (Ferguson made lots of them), as can freebies like Eriksen. Ultimately it is about bringing in the right players, with the right attitude, at the right price, and having a manager capable of bringing the best out of them. Our signings have been mostly "miss" in recent years, but I am hopeful that ETH knows what he wants, and will veto any signings he doesn't want. His track record is pretty good, both at Ajax and so far for us...