Thats like saying that the 12M spend on Martinez is good business just because it is paid in installments. We can give our opinions about the payment method but it doesn't justify some prices that we've paid.
22.5M for Amauri being paid in 10 installments may sound attractive, 2.25M every year seems like a piece of good business but it doesn't take away that you've paid a certain amount for a player. So no matter how you look at it you've paid 22.5M for Amauri. As far as I know a lot of transfers get paid in installment but it doesn't give us more budget. We could still be paying Werder Bremen for Diego's 25M price tag.
No Marintez is completely different because so far he's been a crock and hardly seen the pitch. Diego is a good example though and I agree with you on the fact that we did overspend on some players. Although, we've also had good deals on Krasic and Aquilani.
Pepe at 10m for me is a bit pricey but if he continues to work hard and contribute when he can then he may justify it. Quag again, for me, we overpaid though he could justify his price if he keeps scoring. Same with Melo, Diego, and Amauri - overpaid.
But regardless of all that, the point I was making was that it isn't going to take much out of next years budget to retain what we want from this years squad.