I think the cost is closer to $200 USD per foot of 8 foot wide dock using treated pine decking and PVC posts with a palapa and electricity and water and of course using SS hardware. The lift you can purchase through Captain Sharks. Nobody builds more docks on Ambergris Caye that Armando Graniel.
You can always find someone to do the work for less but there is a cost to that.
Not all docks are created equal so make sure you get everything you need