Below is a recent thread that you may find of interest.
https://www.scubaboard.com/community/threads/suggestions-for-budget-friendly-early-december-dive-vacation.547793/
When comparison shopping be sure to pay attention to what is included in the stated price so that you are comparing apples to apples. For example, most Cayman resorts include the 13% government tax in their listed prices (but not all of them) and many Roatan dive ops do not include the 19% tax in their listings. Other "hidden" charges may include service charges, resort fees, marine park fees, fuel surcharges on liveaboards, etc. And if you dive Nitrox, keep in mind that there may be an upcharge for Nitrox. You also have to include tips for the dive op in your costs.
Liveaboards often seem like rather expensive dive trips but they frequently offer specials, and they may actually be one of the cheapest options when you consider the cost per dive as compared to a land-based trip, but they are usually week-long trips.
Everything is included except tips and you often get to dive the more remote and pristine dive sites. And they won't charge a single supplement if you are willing to share a cabin with a same-gender roommate. The Saba and St. Kitts trip offered by Explorer is one of the less-expensive liveaboards and it is very good diving, and they are offering some special prices in the upcoming months, see the link below. The Aggressor fleet often offers a 30% or greater discount in February for trips throughout the year.
https://www.explorerventures.com/30th-anniversary-sale/
...Grand cayman is also a SW route but that location is pricier I think
Cayman can be very expensive but there are diver-dedicated resorts that offer discounts and specials, and flights and rental cars are usually fairly reasonable. Right now Sunset House is offering a single diver special through the rest of this year on week long trips, see below. The price includes lodging, breakfast, 2 boat dives per day and unlimited shore diving on the house reef.
http://www.sunsethouse.com/specials/single-diver-special.htm
Sunset House is charging $1814 per week for their solo traveler, courtyard room, dive special through Dec 15, which is only $170 more than the price per person for double occupancy ($1642). They usually charge $2287 for single occupancy in a courtyard room with the dive package.
Sunset House Cayman Singles Special!
Tackling Being a Diver as a Single Traveler
$1,813.90 (Single Occupancy only)
Minimum seven night stay & six days of diving
Courtyard rooms only
Subject to availability
Valid for travel until December 15, 2017
Booking Period: Now - December 15, 2017
The Clearly Cayman resorts have small, diver-dedicated, off the beaten path resorts on all 3 Cayman islands, and very good diving. The resorts are all-inclusive and include everything except dive op tips and liquor, unless you buy a package that includes beverages. But the prices are per person double occupancy and there will be a significant single supplement charge if you travel solo. Most of the listed prices are per week but they will pro-rate the fees for shorter trips; see the links below.
Clearly Cayman
http://www.clearlycayman.com/divefree/