You cannot say whether the players brought in under Mourinho have been cheaper or not than what Poch wanted.I'm glad you said that.
The answer explains why we also don't invest in good players anywhere near as much as we should. Simply put it's down to greed for immediate gain with lack of foresight.
Levy didn't want to pay the Wijnaldum money because he thought we could get somebody cheaper in. We couldn't. Result was having to go for Sissoko and having no choice.
Levy thought hiring a new manager would be cheaper than Poch's rebuild. Wrong. Result was a higher paid manager who has brought in 9 players and needs at least 3 or 4 more most likely at high cost.
The most expensive has been Bergwijn which was around £27m while we paid £25m for Sessegnon and £55m for Ndombele.
Personally don't like to focus too much on transfer fees as there are many factors that go into it. At the end of the day, what matters is the quality of players in the squad and we are currently lacking in several positions if we want to be title contenders.
Mourinho, Poch, or any other manager still need quality players to work with.