How do you or anyone else on this forum know what a player is worth to the club?
You have a valid point in there somewhere but the reality is that none of us have a single clue why we pay what we do for any player. People say shit like "oh he's not a £50m player" but what exactly are they basing that on other than what he does on the pitch? It's an oversimplification of a very complex deal.
However, I do think it's perfectly reasonable to question value for money in comparison to other similar players or when we allocate 90% of our summer transfer budget on a single player when some of that money could go towards strengthening other positions.
But, yeah, I do find it tedious when someone has watched a handful of games on TV and decides what a player's transfer fee should be.