If this is going to be a 'forum' event then prizes should be event points for the winners possibly depending on x captures=event point but it needs to be tested in the first place approx. How many flags people might capture in 2 weeks.
You can make the first event with rewards other than event points such as new clothing to convince many to play and see how many times might they capture. Then divide the winner's number into reasonable event point number he could win from that, and so get your capture:event points ratio.
