Google for a website called "vivanuncios" (I can't link to it since I'm a newbie), it has been the best one I've found for online listings in the Yucatan area.
It has a great range of long term rentals and gives you a good idea of prices. I've actively been using it to look for places and to use for reference so I don't get over-quoted when I go on the hunt for a place to rent.