The price can vary by season and sailing, location on the island, and type of cabana, and can be quite expensive IMO. Over the water cabanas at Coco Beach club, for example, can exceed $3,000. (They typically would be for larger families or 3-4 couples traveling together). Hideaway Beach can run $1,500+. The ones on South Beach are far less costly but can still be in the $300+ range. Note also that Coco Beach Club and Hideaway Bay are private areas and carry an upcharge, which would be included in the cabana cost.
The only way to determine an exact cost would be through the cruise planner with a booked cruise. But be prepared IMO for sticker shock!
There are other alternatives with large umbrellas in most of those locations as well as at Oasis Lagoon and other spots, which are n/c on a first come basis.