With the craziness going around the football player transfers sphere and almost every deal completed being such a cliché, I see no reason why Atalanta shouldn't stand their ground for the 90m €uros prize tag they want for their young striker, If Man United could have paid for Antony a sum of £82m+, Harry Maguire £80m+ and over £75m Romelu Lukaku, I really don't see the problem with such fee for Rasmus Højlund if they indeed need him, At least in this window we've seen a club pay much more than that for an older CDM and another club paying over that as well for a CB, so Man United should stopping nagging already, accept and complete the deal if it's their priority, huh! 
Actually the price of players cannot be compared with other players unless they have the same achievements and contributions, like Rasmus Højlund his contribution to Atalanta has not been as impressive he only scored 9 goals in 32 matches it's just that he's still young so Manchester United think that they can increase his potential. Almost a year Atalanta brought in Rasmus Højlund with a transfer fee of 17.2 million meaning that Atalanta has made a high profit if they release it at 75-80 million especially now that Atalanta also needs fresh funds. I believe in the end Manchester United will pay 80 million or more for Rasmus Højlund.
I don't think that's how the transfer windows works any longer, a player might not have done pretty well in a season, but that isn't what makes determines their fee, what has Aurélien Tchouaméni spectacularly done before Real Madrid splashed heavily on him, the same goes for Mbappe transfer from Monaco to PSG, Monaco has team were incredible that year and Mbappe should huge prospect too, but was he supposed to be what 180m back then, I don't think so.
Anthony also only scored 9 and 8 goals in his last two season at Ajax playing 32 and 28 games respectively in the process, but that didn't stop Man United from paying £82m for him. I don't think the fee is too much for a player of his prospect and skill set, Hojlund would be what Man United was expecting of Werghoust imo, and as for last season he had only started in 17 of those 32 games you mentioned.