Jane is right on track. When you say "Kona coast" are you thinking right in Kailua-Kona or up north along the Kohala Coast? Something on Alii Drive in Kona will rent for very different rates than a resort property at Hualalai or Mauna Kea or a house near the ocean at Puako. If the property you are looking at is not currently being rented, you would also have to count on a period of years to build up a clientele for it. Once you've narrowed down the property, your agent should have an idea of which companies do rental management in that area, and you could contact them for information on rates. Also be sure that if there are CC&Rs in the community where you are buying, that they permit nightly rentals.
And if you aren't doing the rental management yourself, be sure to consider that the rental companies take, on average, about half of the nightly rental income!
Good luck with your purchase.
Beth Thoma Robinson R(S)