The problem is that a lot of thought and preparation has to go in to spending anything over £25-30m on one player. If the price that we are talking about is anywhere near the £45m mark, and then 5 years worth of wages (£100k p/w) is added to that, we are talking a £70m transfer. That's an enormous investment.
Sure, Real Madrid are willing to invest that much, but look at who they are investing it on, and then notice who they are haggling over. This is something that should be noted. Real Madrid are willing to spend almost any amount on players that are already considered as one of the best in the world, as well as also having a huge commercial value, yet they suddenly begin to look almost frugal when attempting to sign players like Xabi Alonso.
That's because he has little or no commercial value, so his only real value is on the football pitch, helping the team to win trophies. If he isn't absolutely the best player in the world in his position, Real Madrid would rather move on to another player that is of a similar level, than overspend on one with little or no commercial value.
Aguero is potentially one of the best players in the world, which means that his commercial value is potentially very high. But he isn't there yet, in my opinion, so it would be some risk to spend anything more than £30m on him, which is coincidentally what Madrid spent on Benzema. But you won't get Aguero for £30m, as Atletico had to fork out nearly £20m to sign him when he was 18 years of age. Given that he has proven that he can play in Europe over the last 2 seasons, and has shown plenty of signs that he will develop in to one of the best players in the world, it is not unreasonable of them to want close to £50m for him.
But it would be unreasonable to spend that much on him, in my opinion.