To fully answer this would take quite a while. What I would say is the true cost of signing a player should always be fully understood, no? If Mbappe signed at Real Madrid instead of PSG was that truly a "free" signing? Is Haaland's cost comparable with with other £60m signings? For Haaland, how much do you bet if he's doing well we're going to hear non-stop next season the £60m that was paid and what a deal it was without mentioning the other £150m+ also being paid?
Wages across clubs and leagues are not standardized as well so to assume what Barca pays their £50m signing and what Arsenal pay their £50m signing are the same is clearly not true when you just look at their respective wage bills.
Diaz for instance, I wouldn't make any assumptions. LFC under FSG have had no problem paying Agent fees and our wage bill is top 5 in the world. Diaz isn't going to be on peanuts even if it may pale in comparison to what Haaland or Mbappe is getting.
All of this as you're noting is due to the lack of transparency aside from the transfer fee itself so that's what people go on. You can infer it by the books of the club the following year but it's never cut and dry, hopefully it starts to be reported more regularly though as it gives fans a better understanding of what something is or isn't truly costing.
This further highlights my point.
Mbappe and Haaland are always used but they are incredibly unique cases. They've been built up to be the next Ronaldo and Messi already and this has no doubt been used to leverage their positions and demand these huge wages, how many other players have the bargaining position to do this? There will be VERY few. The 2 of them are the exception, not the rule surely?
Take Tchouameni for example, he's not someone who is going to be demanding that kind of money or even remotely close, who's to say he's going to even be asking for more than many of our existing players? Who is his agent and how much would we be looking at in agents fees? What is his current wage? It's all good and well saying clubs need to show more transparency but how can a discussion happen without that? It again comes back to people making wild assumptions based on little factual information which negates any worthwhile discussion on the 'total cost' of a player. Happy to be proven wrong and/or educated it but you've mentioned fees in Tchouamenis case and even compared it directly with that of Bellingham, what is this based on? Is this a good chance of being highly accurate due to being based on factual information or just they're both young and highly rated so same financial package is a given?
Part of the reason these discussions aren't really needed is because the club is well run we wont be bringing in signings who obliterate the wage structure. Diaz may not be on peanuts, but none of the footballers here are, his wages may however be incredibly disproportionate to his ability (solely in a footballing sense) he wouldn't be the first red you can say this about.
Talking about other factors in deals can be worthwhile but a lot of the information isn't even available, there can't realistically be a push for this to happen with the majority of the information not even available. It would simply lead to shouting down of other fans who want new signings stating they are showing ignorance and expecting too much and/or shouting down of fans who speculate over these financials without having all the necessary info.
