Before 2011, Aoki had a cumulative batting line of .302/.372/.482. For the 2011, when the NPB introduced the new ball- more similar to the MLB regulation ball- he hit .297/.354/.433.
The four-time All-Star has a lone (Mitsui) Golden Glove Award to his name, received in 2008. He isn't completely lost with his glove, but he has been known to get sloppy at times, especially with his throwing. His athleticism makes up for a lot of his shortcomings, and as a result, Nakajima possesses good range, a strong arm, and a propensity to make highlight reel catches (sounds a little bit like David Wright). In addition, he has some speed, and has stolen as many as 25 bases in a single season. His caught stealing percentage, historically isn't that bad, as he is 134/39 in his career, but looking at a breakdown of his stolen bases over the last three years (21/2, 15/5, 20/12), there's a lot of room for improvement that isn't likely to come. He is 29 currently, and will turn 30 in July.
Nakajima hits for both average, and power. Since becoming a full-time player in 2004, he has hit no less than 12 home runs in a single-season, which is pretty good for a shortstop- he peaked in 2004, with 27 home runs, which is pretty extraordinary.
I don't think that the Mets are going to be in on the bidding at all, as it would be a clear signal that they aren't going to re-sign Reyes, and I don't think the team has fully made up it's mind on doing that just yet. That aside, the bidding on Nakajima and eventual contract will probably be somewhat similar to that of his former teammate Tsuyoshi Nishioka. The winning bid will likely be between $5 million and $10 million, and his contract will probably be for about the same amount of time, three or four years, depending on options. Nakajima will probably make more annually than Nishioka is, because of his theoretical home run power- maybe $5 million a year?
If the Mets weren't in the Jose Reyes at all, or decided to fold, Nakajima might be an interesting alternative at shortstop to Ruben Tejada, who would be the most natural incumbent should Reyes not come back in 2012. His play, if he put up similar numbers to the .297/.354/.433 batting line he had with the new NPB-regulation ball, he'd rank among the elite shortstops of the National League. Accounting for some regression because of better competition (especially in the power department), even if he hit .275/.350/.400, he'd still be somewhere in the middle of the pack. To put it another way, he'd be giving you Jimmy Rollins numbers, without all of the money Jimmy Rollins makes.