Let's say that when Prior pitches, we have around a .667 winning chance(41-23, about that) for the game, compared to .400(Career record 12-18 for Bedard) chance to win.  That's a difference of .267.  Assuming 25 decisions, that comes out to a decision for 10-15 for Bedard and about 16-9 for Prior.  That's a difference of Prior winning six games that bedard lost.  Tejada would, by my estimation, win us around 10-15 that we would lose otherwise.  
Sure, six games doesn't seem like a lot(though that's the difference between 79-83 this year and 85-77, big difference), but the fact remains that pitching is not helpful to go deep in the playoffs, it's necessary and a prerequisite.  Fact is, like my dad always says, Good pitching stops good hitting.  I don't think that great hitters can hit the best pitchers on their better days.
I think that Prior should be sticking around for three reasons:

1.  I think the injury prone thing is a bunch of bull.  Last year he got hit on the arm, and he did not have much trouble that I know about at SC.  I don't think his motion is dangerous or maligned with problems that will reinjure his arm throughout his career.
   
2.  If he is healthy, he can throw complete games and go deep in innings without losing much of his edge, something that a lot of pitchers don't have the ability to do.  He eases the pressure on the bullpen by not coming out early very often, and allows the bullpen days off once in a while, helping them to help the team more, and stay off the DL.  He eases the pressure on the offense too.
   
3.  I think that Tejada has peaked in the last few years, but Prior has certainly not.  I guess maybe I'm just hopeful that he can stay healthy, and pessimistic that Tejada can keep this pace up(he was worse in every major offensive category last year, which may not be a fair comparison, because almost everybody was worse than his 2004 stats.), which I see slipping a little next year.  We're not going to win a championship with the current lineup, so in two or three years the trade could look dumb from our standpoint because by then Prior will be entering his prime, and Tejada could very well be on the decline.
   

1.  Tejada is the epitome of durability (Average of almost 162 games/season since 1999).  But that may change with age, as it often does.
   
2.  Pitching is bigger risk and harder to find; consistently great pitchers are hard to find and consistently great hitters are not quite as hard.  For every Roger Clemens or Curt Shilling there are about 5 Albert Belles, Adrian Beltres, and Juan Cruz's(40 batters this year with 26+ HR, but only 23 pitchers with 15+ Wins.)