There's context to be added when it comes to signing from the championship and what you pay. Latte Lath is nowhere near a £30mill player (imo). However Boro are in the playoff spots and selling him could see their chance of promotion, however slim you might think they are, slip away even if the replacement is decent but not quite up to his level. Selling him may derail their season, so is it worth it to them for less?
Haji Wright and Borja Sainz play for sides unlikely to reach the playoffs and unlikely to go down so with them you're probably getting a more realistic valuation based in the players worth in isolation.