If we could have found a top keeper for 10m 2-3 years ago then I'd definitely have signed one instead. The problem is top goalkeepers do not grow on trees, you only have to look at some of the clowns we were linked with over the summer to see that, Hart and Reina were probably the only two that you could consider 'top keepers' and even with them we had no real chance of signing them.
The same applies for the other positions as well. If you go for a young player they could turn into a Cesc or van Persie who we'd otherwise have no real chance of signing either due to price but more likely because they'd be at another big club.
That's not to say you don't sign ready made players when you can, my point is that there is more to signing youth than just saving money.