Money buys talent, not wins. The Yankees and other such teams are fortunate enough to have gobs of talent on their rosters at all times. However, a stable team must be built with some sort of foundation in place in order to maintain their winning ways. That's why the Red Sox stayed competitive over the years; they made shrewd trades, signed the right free agents, and compiled an impressive core of young pitching. Their payroll also allowed them to get Dice-K, Manny Ramirez, Curt Schilling, and J.D. Drew. As much as I dislike Boston, they're managing their team the right way.
Then you have a team like my beloved Cardinals. I still refuse to acknowledge their 2006 World Series win as a fluke because, well, it wasn't! 2006 was merely the last hurrah for an old core of players who were ready to break down (see: Edmonds, Rolen, Eckstein, Weaver, etc.). Now they are paying for years of farm system neglect.
You must have both a top 10 payroll and a self-renewing talent base in order to stay competitive in this league. Either/or will keep you around, but not very long.