I can't give out that information but let's just look at the winners since adding the contribution level.
July 4th winner: Scratch with 296 posts, Level 14
July 21st winner: FreqE1 with 315 posts, Level 11
July 28th winner: Bushwickbill with 110 posts, Level 12
Sept. 3rd winner: Conky with 36 posts, Level 6
Nov. 16th winner: RJones with 5 posts, Level 11
Looks pretty obvious to me that the more you contribute the better your chances are. The more sweepstakes we have the more clear this will become as it will start to favour the regulars even more.
You also have to remember that contributions is not only posts or threads started. It takes into account how long you have been a registered member, how many people you have referred. How many sticky posts you have. By FAR, the best way to increase you chances is to refer more people and create excellent posts that we decide to sticky as you get 100 points for each.
At the end of the day, I hope people post because they want to. The sweepstakes should just be a bonus.