Pretty much what I said earlier. If we had De Gea, we'd have won the CL last season. For some reason, keepers have always been under priced compared to other areas of the pitch. Yes forwards generally will be more expensive as they score you the goals that win game, but a keeper and defence are the one that make sure you don't lose them as well.
I also think that 15 years ago, defending was easier. You didn't need to be as good a defender because you could get away with a lot more. More fouling, shirt pulling, give the forward a kick early doors without getting a yellow. Nowadays, any kind of touch is a foul, and tackle is a yellow so defending has got a lot harder which means having really good defenders and keepers is far more important which means the very best are now catching up price wise with the rest of the positions.