I would recommend contacting Awesome Caribbean Weddings. I haven't used them myself, but have heard wonderful things about them. Their website is Awesome Caribbean Weddings - Your Wedding in St. Lucia, St. Lucia wedding planners for unique Caribbean weddings
As for affordability, the villa we rented (we split the cost with our guests) came with cooks, so we had a lot of homemade meals which saved a ton of money. (We chose to do a villa because we wanted to have a place for just us and our guests. We also had a 2nd villa a few minutes away.) For the wedding, we got married on Pigeon Island, had the reception dinner (cocktail hour paid per drink, 8 different canapes, soup or salad, and a choice of 3 entrees) at Jacques Waterfront Dining for about $3500, then went back to the villa for cake and dancing. We had 34 people.
Probably the biggest budget blower in St. Lucia is the taxis. It's definitely expensive to get around, but renting a car wasn't an option for us because I wasn't going to tackle left-hand side driving!
Check out Coco Palm and Harmony Suites for affordable hotels. They're so close to everything. My parents stayed at Harmony for a few days and really enjoyed it.
I hope I'm being more helpful than confusing! And if you really want AI, don't let me discourage you from that!