I have, in the last few weeks, assisted two of my customers in their purchases just north of Habaneros in the Robles and Palm Bay areas of the island. My two most recent customers acted quickly and are now proud owners of caribbean beachfront. Several others decided to "mull it over" and are now upset that the properties they were interested in are now off the market.
The homework is pretty easy to do on pricing, simply see the real estate link on this website's home page and compare the different agency's pricing for properties in the area you're interested in. Ambergris Caye has plenty of reputable real estate agencies to choose from and since they're paid by the seller I would suggest you take advantage of their expertise in the local market. I can tell you that both of the recent sales that I assisted in went within 5% of asking price and one was full price. I know it never hurts to ask but don't expect big discounts off the asking prices here. I agree with tincup that the larger parcels will afford you the most opportunity for future resale and development.