Moustfa,
Many people including me don't like to participate in contests type of forums as it's always on luck. If luck is not on our way, then all our efforts will be in vain. So people always prefer PTP over contests.

I think you would be better off if you keep low rates per post, than having contest. Contests is a secondary thing for us. I may participate in your forum because I know you, and you are like my friend. But it's tough for us to introduce new, unknown members in your forum.