Good points; I'd been wondering the same thing. GMs really seem to forget the old adage of buy-low/sell-high.
A telling example for me last year was the Yankees' decision to sign Sheffield rather than Vladie. Sheff had his best season and was something like 35 and under a steroid investigation. Vlad was about 28, and had an off year (admittedly for back-related reasons). For the same money they could've had a better fielder and younger player, but they bought high, just like with Brown and Vazquez. And before that they bought the reigning MVP, Giambi, at the height of his value.
Other teams 'buy low' but STILL pay full price, like the Diamondbacks decision to sign Glaus, or their overpayment of Russ Ortiz (who had a sweet 2003). It's really puzzling. If you look at the guys the Red Sox signed -- Mientciewicz, Cabrera, Bellhorn, Mantei, Schilling -- they're all guys who got signed one year AFTER a great year, but who had an off year preceding the time when the Sox grabbed them. Other than Schilling, they're younger too (32 or less, still in roughly the prime range), so any performance decline most likely isn't attributable to age. Notice that Theo is trying to trade Manny again? Manny is at the height of his value--world series MVP. He's trying to sell high.
Needless to say, I like the Theo Epstein model --even if it doesn't work in a given case, it's cheaper-- and I don't get what the Yankees and Diamondbacks are doing. Beltre and Beltran will get huge money and they'll earn some of it back, but it's unlikely in my mind that they'll perform at that level consistentl. Some teams get $0.75 on every dollar they spend; others get $1.25.