The argument that you should not be concerned how much a player costs because "it's not your money" is one of the daftest things I've seen on here recently.
Unless you believe we have a limitless budget and/or don't care about FFPR, then logically you must accept that we have a transfer budget.
If our budget is £80 million and we spend all of that on one player, that people think is worth £5 million, then of course you would be right to query the amount spent; not because it was your money but because it meant your club had wasted £75 million that could be better spent elsewhere.
You even have people expressing an opinion and then attaching the caveat " but I suppose it's not my money" for fear of being brought to task by the posters who think this is actually a valid point!
If you think a player is a waste of money just say so, you might have a good point, whereas the reasoning that it doesn't matter because its not your money so stop moaning shows a complete misunderstanding of how the business of running a modern football club works.