well i agree that the Sox dont have a ton of homegrown talent (you forgot to mention Trot), thats not how it works with free agency. Renteria was the best SS on the market so the Sox went out and got him. He plays great D and is a pretty good hitter - he will come around.
As for the homegrown talent issue - homegrown is a useless term. If you trade say Slocumb to the Mariners for two prospects who have never played in the pros do VTek and Lowe count as homegrown? And if not then Schilling, Bagwell, Brady Anderson, and Pavano count as homegrown for us that were traded to other teams. And then theres the players that came up with the Sox and left - Clemens, Mo Vaughn, Valentin, Nomar. You can't critisize the Sox for signing FA's when a lot of good players left the Sox for the same reason.
Then theres the issue of signing players that no one else wants - Ortiz and Mueller were paid next to nothing to play for the Sox because no one else wanted them. Should the Sox be looked down upon for signing the likes of Ortiz to a $1.5 million contract?
Obviously more "homegrown" talent would be good to keep payroll down but the term is useless in constructing a baseball team. Prospects are traded all the time and free agents move around every year. And the Yankees only homegrown players all make $10+ million a year now (Jeter, Mo, Posada, Bernie)