It means exactly what I've stated - if a club spends 75M pounds on a player - said player is expected to regularly start and contribute big time towards the team's success.
75M pounds is not being spent on some sort of squad/backup/reserve/2nd-rate player.
If we were to sell Sterling and/or Sane, how much would you expect them to go for? I'd rate both at at least £75m in today's market.