Very few islands ever come on the market in Mexico and when they do they are often leasehold properties (as opposed to freehold). However, one island that recently came on the market and that is sure to catch the attention of the world is San Jose Island. The island is rumored to be about 2 million acres in size and is on the market for $150M.  Private Islands Online the agency that is advertising the island has an extensive description about the island that you can read here. From the description it appears that the present plan is to develop the island as an ecopark.