From what I remember seeing of people mentioning costs to rent the cabanas, I believe the cost does vary a bit depending on the season/demand. Peak summer season is gonna cost you more than some of the less crowded or colder times of year.
We rented a Cabana for our stay in July this year. It was $50.00 for the entire day from 9am until the pools close. I called two weeks prior to our arrival date and reserved it. I was able to pick which pool to be near and which Cabana I would like.
We got one near the pool by Tower 4 as we wanted a quieter area. Next time tho we may get one near Tower 6 as my 17 year old loved the slide at the Pirate ship.