The prices start dropping in mid to late April, then further in June or July until mid-Dec. (I had much the same concern and spent some time plugging various dates into the website, they have a print catalog that's soooo much faster for that sort of thing...
oh well) They have five price levels actually, though some levels are farther apart than others.
For us, that early summer price break in mid-April is sufficient for future charter plans. Since the plans need to be made somewhat long term (in order to co-ordinate with friends and family), the risk of disrupted plans due to tropical systems doesn't interest me (though I know the odds are slim of encountering such a disruption).
Sorry to be long-winded, but I wanted to give an idea of a thought process you might go through, not just a short answer.
[edit: was off a bit re the price breaks, July-Oct is lowest, with April-June and Nov-Dec 19 next lowest, Christmas through mid April has some relatively minor price variations. I still think that for those that have to travel (read: fly > 3 hours) to get there, the April/May time would be best. But then again, I only been there twice, so you mileage will most definitely vary!
re-edit: I think I goofed something up in the edit and I don't have the brochure/catalog/price list in front of me. Sorry, however a lot of the fun IS the tootling around on their website...]