I think your right to think that May is cheaper. i know around spring break time its a bit more expensive down there. I think that october ish is a cheaper time as well, but keep in mind that time of year can come with hurricanes. My guests paid around 1500 pp at Dreams Tulum, which includes airfare, All inclusive resort, and transfers to and from the resort, for 7 days, and 1200 or less for 5 days. hope that helps:) Have you been in touch with a TA?