The AP has the scoop, and it's kind of complex, but that's why CTRL-C and CTRL-V were invented:

Griffey can earn $250,000 each for 450 and then 500 plate appearances. If Seattle draws 2 million fans this season, he would get $100,000 more for 300 plate appearances, $200,000 more for 350 plate appearances and $200,000 more for 400 plate appearances.

The money increases if more fans come. Griffey gets an additional $200,000 each if Seattle's paid attendance is 2.1 million and he has 300 plate appearances; 2.15 million fans come and he has 350 plate appearances; 2.2 million come and he has 400 plate appearances; 2.25 million come with 450 plate appearances; and 2.3 million fans are coupled with 500 plate appearances.
...
Additionally, if he is on the major league roster through July 31, Griffey gets $50,000 each for paid attendance of 2.35 million, 2.4 million, 2.45 million, 2.5 million, 2.55 million and 2.6 million.

If he is on the major league roster through the last day of the regular season, he gets $100,000 each for attendance of 2.65 million, 2.7 million, 2.75 million, 2.8 million, 2.85 million, 2.95 million and 3 million.

Those roster bonuses would be prorated for any days Griffey spends on the disabled list.

The deal has a base of $2m and a maximum value of $5m, which Griffey can only earn if (A) he collects 500 plate appearances, (B) he doesn't go on the DL, and (C) the team draws 3,000,000 fans. Which is great for us, because if 3,000,000 people flock out to Safeco, presumably something is going incredibly well, and the team won't mind spending a little extra money. I love this contract. Not that it really makes much of a difference, but with the way it's written, there's pretty much no way for the team to lose. Even if Griffey suffers a catastrophic injury before the season begins, the team will have already made a good chunk of the guaranteed $2m investment in Griffey-related revenue.

The whole attendance incentives thing still sounds weird to me. A quick look at Cot's doesn't bring up any names with similar deals, and according to Darren Rovell, the last three players to have this sort of clause were Roger Clemens in 2004, Kenny Lofton in 2002, and Mark McGwire in 1997. So it's definitely unusual. Also, Kenny Lofton? The year before Lofton signed that contract, he hit .261 with 14 homers and 16 steals. What were the White Sox going for?

Man 1: Hey, you wanna turn that off and come out to the bar?
Man 2: Can't. Kenny Lofton's playing.

Kenny Lofton is such a popular attraction that, after posting a solid 2007 at the age of 40, he couldn't find work, and now nobody really knows what he's doing. That was one heck of a clause. I don't know which side insisted upon its inclusion, but I do know which side still thinks it's funny.