Since the start of the Klopp era, I reckon there's 5 types of players that make it into our first team
Academy players
U21 players we sign as a cheap-ish gamble (Elliott, Doak, Leoni, Carvahlo) & can at least recoup our costs if they don't work out.
21-25yo established players that we think can immediately play first team & that we'll get 6-10 years out of. The vast majority of our signings. We generally don't care about the resale value of these players 'cos we hope they'll pay for themselves by spending most of their top level career with us. Selling Diaz at 28 is pretty rare in our system - but we'd just reached an impasse where he wanted more that we we'd offered him. Doesn't happen often
Big money players over 25 (Virg, Isak, Ali) - we've got to think they're a sure thing but will occasionally spend big on this type of player. Again we're not generally looking at resale value with these players when we buy them
The occasional cheap/free older squad player (Endo, Chiesa, Milner)