It isn't so much how much they spend on free agents. Rather, it's the endless capacity to gamble on injured players and eat bad contracts where the Red Sox use their financial advantage. Lowell in 2004/05 is the one where eating a bad contract worked out the best. They ate $11m on Rentaria, $13m on Lugo since 2005 at one position. They can eat Guz's contract if they need to next year.
After this year, they have to reassess their ability to count on rehabbing players, especially pitchers, for depth. Between Wade Miller (?), Bartolo Colon, Schilling 2008, Penny, and Smoltz, that's been mostly negative returns. They might have gotten 10 wins out of all those experiments, with more than half coming form Penny and his 5+ ERA.
In contrast to their willingness to eat their own and others overpays, they tend not to overpay on multi-year big bucks free agents. The current ownership has only given out 2 contracts over $50m in total payout (Dice-K and Drew). They bid, like on Teixeira, but they have limits.
