West Ham's position means they'll always pay over the odds for players with any kind of reputation. That's just how it is.
He scored 19 goals in 27 appearances last season for Frankfurt and is supposedly a good team player with more to his game than just finishing. He can't squeeze past Mbappe or Griezmann but those are players on a different level altogether. West Ham are currently a mid-table team at best with aspirations to become a consistent 7th-8th placed one. Haller's at least at that level by the sound of things.