I voted no.
From a personal perspective: I'm not interested in prizes, I just play for fun and would not enter if it had a fee.
From a general perspective: You're probably encouraged by the amount of signups since the introduction of the new FF1 format, but I think that if you introduce a fee a lot of these new (and also older) players will quit the game and it will also discourage potential members from signing up and entering. At this point in time it will only make you lose members and go against what you're trying to accomplish, which is more members actively participating on the website.
If you just let it run for a couple of seasons, let it gain a repuation as a fun and well run game and then look again at how many new members we've got since the introduction of the new FF1 format and how many of those actively participate on the site, only then do I think you can make a fair decision on whether to introduce/change it to a paid format with prizes (perhaps hold another poll like this to see how many members you'd lose) and if it's worth the extra time. Also you should keep in mind that you will most likely not be able to make 100% sure that no one is cheating...
In short: no at this point in time; maybe in the future.