Gardner's offensive power is as ugly as it gets, but his slugging is artificially low relative to his actual offensive value because he's an excellent base stealer. For his career he is 39 for 45 in steal attempts, or 87%, which is as close to automatic as you'll get. If we add his 39 successful steals to his total bases and subtract his 6 failures, his slugging becomes .440 for his career. This approach ignores the jump from .299 to .379 his slugging took between 2008 and 2009. If he maintains his 2009 levels of production in 2010, then his slugging will be closer to .464 in 2010, if we count his steals among his total bases.
Gardner has also displayed an improved OBP between 2008 and 2009. He improved his performance to .345 in 2009, from .283 in 2008. During spring training 2009 Joe Girardi would rave about Gardner's speed on the base path, which caused this Yankee fan to scream at the television since, you know, Gardner never got on base in 2008. In 2009 he improved dramatically, to the point that he only needs to maintain his '09 pace this season to add value to the lineup.
Gardner is also a dramatic upgrade in left field from Johnny Damon. His 2009 UZR/150 was 15.4 in CF, which is fantastic. He started 15 games in LF in 2008, and posted a UZR/150 of 36.8, which is other-worldly. Compare this to Johnny Damon, who posted UZR/150's of 11.6 and -12.1 in 2008 and 2009, respectively, in left. That's an incredible decline.
According to Baseball Prospectus, Johnny Damon posted a VORP of 39.3 in 2009, while Gardner posted an 11.4. VORP is restricted to offense, and is adjusted for playing time. Damon made 626 plate appearances in 2009 versus Gardner's 284. Let's assume that Gardner doesn't get injured in 2010, and makes twice as many plate appearances. This is fewer than Damon saw in 2009, but the team has indicated a willingness to platoon for Gardner. In that case, we can roughly double Gardner's VORP to get a sense of his true value over the course of a season. He would be worth 22.8 runs.
UZR/150 tries to measure the number of runs a player saved (or would save) over 150 games at a given position in the field. If we take Damon's actual VORP, or runs produced offensively in 2009, of 39.3 and add his UZR/150 of -12.1 we find that on both sides of the ball Damon was worth 27.2 runs in 2009. He put up great numbers on offense, and then gave some of that back on defense.
Turning our sites on Gardner's assumed production for 2010, we begin with his estimated VORP of 22.8 and add his UZR/150 of 15.4. This brings us to 38.2 runs produced or saved, over about 150 games. Put another way, its easy to project a scenario in which Brett Gardner is BETTER than Johnny Damon next season, by 10 runs exactly.
This leans a bit more towards a "best case" than I would actually like. I am assuming that Gardner will not get hurt, will maintain his 2009 production levels, and won't display volatility in the field, which is common when using UZR. That's a lot of ifs for a young player. Johnny Damon provides a lot more data to work with. That said, the numbers don't lie. Brett Gardner's 2009 production on offense and defense over a full season was actually worth more than Johnny Damon's.
I put together these estimates in about an hour, using a laptop and a calculator. My conclusion is similar to that of the entire New York Yankees organization. I like to imagine they spent more time and effort figuring out what to do with at least $2 million.