Per the FanGraphs stat Wins Above Replacement, Cliff Lee has pitched like someone likely to produce 3.6 more wins for the team than a scrub, AAA call-up type of player would have in the same starts. Adrian Beltre, meanwhile, has put forth 3.3 wins per the same stat above a replacement-level third baseman.
Pretty close, and Cliff Lee has the slight edge. However, Cliff Lee faces NL lineups, which are not only weaker, but feature pitchers hitting a few times a game. Even if he got to face the Mariners, Angels, and A's nearly 60 times a season, his WAR would likely be lower if he pitched in the American League.
Another, similar, form of WAR found at Baseball-Reference has Lee at 3.8 and Beltre at 3.3. Again, close, even though Lee gets to play in the NL.
Just one very simple way of measuring things, and it's only from a little more than three months of data, but, so far, there's a pretty good argument that the Rangers have been as well, if not better, off with Adrian Beltre in 2011 than they would have been with Cliff Lee. Not going to stake my reputation on that holding up, but that's also not out of the question. Lee has been great this season, but so has the Rangers' new third baseman, and for at least $40 million less than the lefty would have cost.
Fun with stats!
Hat tip to Lone Star Ball poster matchst1ck for bringing this up.