Well, sure, and obviously that's what the forum is for - commenting on current events in the club, and that's all fine and dandy. But I don't see why someone should be worried about whether we paid 25+20 or 35+15 or whatever (as long as it's not something horrific like over 100 million). These are not figures that will endanger the financial wellbeing of the club. Or to put it differently, I don't see why we as supporters should care more about the exact figure than we should about whether the player will be good. Too often I see stuff along the lines of "if we can get him under ___M, I'll be happy". I mean, what's in it for you? You'll be happy if he plays well, and you'll be sad if he doesn't. Whether the club paid 25 or 35 million for the player shouldn't be of much importance to you when you're watching him play.
Obviously you don't care about 10 million euros and that's fine, but that doesn't mean you can tell everyone else they shouldn't care about that amount of money. As I said before, people are very invested in their clubs, and buying a new player is not an exact science. It's all about risk versus reward. Fans want the maximum benefit for the minimum cost, and we all have vastly different opinions about players. Go and have a look in the threads about Bravo and ter Stegen. Those are Barcelona players who we all presumably watch on a regular basis, and yet some fans have polar opposite opinions about them. So, why would you expect there to be anything close to a consensus when it comes to potential signings?
Fans look at how much we've spent on the likes of Henrique, Song, or Afellay for example, they add it up and say we could have gotten a great player for this amount. Vermaelen was obviously a high risk transfer, and maybe fans would have been okay with risking €5M on him. But they weren't happy about spending €15M. That's €10M that in their mind could have gone towards filling another position. No-one likes to feel cheated, and small amounts add up. Look at the Douglas thread to see how a relatively low-cost signing created a lot of angst.
So, like I said, I think it's perfectly normal for fans to care about how much we're spending on players. Even when we're only talking about small sums of money.