A quick scouting report on the 29-year-old Aoki (he turns 30 in January) tells us that he is talented in most facets of the game; he's a line-drive hitter, he sprays the ball around. He has something like that Ichiro step-swing, but manages to keep the head of the bat in the zone and really square up the ball. He shows flashes of power, and hit double-digit home runs from 2006-2010, with 20 in 2007. He is a pretty disciplined hitter, walking plenty, about as often as he strikes out. He can run a little bit, averaging 26 steals at a very good .75 rate. He displays good instincts and great range in center, but a weak arm may limit him to left field.
Let's assume that his fielding will translate into average-plus defense in the majors, so that the main issue is his bat. There are a couple of questions here.
The first is his 2011 season. For the four previous years he put up a slashline (AVG/OBP/SLG) of .339/.421/.497 - very healthy. But in 2011, his power seemed to desert him, as he hit 4 home runs and put up a .292/.358/.360, a sharp drop from his usual MVP-type performance. This was his age-29 season, so it shouldn't be the harbinger of things to come, as he should still be in his peak years, but aging is unpredictable, and it is impossible to tell.
The second is how well his hitting will translate to MLB. The guy can hit, and I expect him to hit MLB pitching, but the question is how much. Using his last three years and Bill James' Favourite Toy metric, I can predict a slashline for Aoki of .316/.391/.424, pretty good numbers, but those are in the Central League.
Just for the sake of this exercise, let's apply the Ichiro conversion to Aoki. From 1998-2000, Ichiro batted .363/.427/.510 with Orix, then with Seattle from 2001-2003 he hit .328/.374/.439. Ichiro was in his mid-20s, in his prime, so the decrease was almost completely from the league change.
If we apply that same level of conversion (0.904/0.874/0.861) to Aoki, then he is a .286/.342/.366 hitter with 10+ steal potential, a passable #2 hitter or a bottom of the lineup hitter. He is a starting MLB player, but not somebody you're going to break the bank for in posting.
How do we value that? Well, with the bat, that slashline, over 150 games, is worth -11 runs in the American League. His baserunning is worth 2 runs, and his defense is going to be worth probably less than five runs, as he is likely an average center fielder or a very good left fielder. Add it all up and you get a 1.8 WAR player. He should help a team out as a role player, but he is probably not an average major-leaguer.
If I am a GM with an outfield need I am willing to commit two years and $15-20 MM to Aoki, including the posting fee. Any more is probably overspending, and probably by quite a bit.