Hard to make an apples to apples comparison, but we were 5 people on a Lagoon 40 out of TMM last year for a week.
Mooring balls were $30-35/night.
We used $236 in fuel for the week.
Filled with water in Cane Garden Bay - $12 (don't really know how much that bought us) and filled with water for free at Leverick (water is complimentary when you reserve a slip - slip is only a few $ more than a mooring ball - highly recommend a slip)
Dont know about cheapest, but easiest place to fill with fuel before returning to TMM is the Delta fuel dock in Road Town. It's on the right side of the bay as you're heading in. It opens at 7:30 am I believe.

Julie
PS: Where are you in Cincy? We used to live in Montgomery :-)