This can be a tough question believe it or not. It all depends on you. Every season has its' pros and cons. Roughly May to October is the low season and rates are at their least expensive. But, you also have to put up with some mosquitoes.... but then again, it all depends where you stay. The town of San Pedro sprays and they aren't too bad actually.
The high season roughly runs from November to April, with Peak season running mid-December to usually the day after New Year's.
Personally, I always enjoyed the high season.