Please, note that I am no longer a builder or inspector, but love selling Real Estate.
http://www.sarasotarealtyteam.com - on the main page look for our preferred vendors. I have others that I would suggest for the islands.
Kent Ellermets, Realtor
The Soda Group
Keller Williams Sarasota-Lakewood Ranch
That cost will vary greatly depending on materials, the current construction of the lanai, and the contractor you choose. It's not possible to estimate a cost per square foot over the internet (and that cost isn't a function of real estate) but you can certainly have a licensed contractor meet with you during your due diligence period.
Please note that, under most standard real estate contracts, there won't be a provision to allow you to cancel the contract based on that estimate. If your decision hinges on that cost, make sure that you provide for the estimate in the contract.
Re/Max Alliance Group
GRI, Certified Distressed Property Expert