I'd suggest signing up at
www.travelzoo.com for their Top 20 deals. Every Wednesday they come out with a listing of the 20 best travel deals for that week. You have to be ready to buy but the deals can be VERY good.
Also try
www.cheapcaribbean.com
Your budget is really only allowing for $300 airfare pp and $135/nt for hotel. For a weekend in the summer (and being only a month away) flying from STL or MEM, that's really not much, especially on the airfare side. Perhaps Priceline can help with the hotel once you find an airfare that suits you.
Good luck and have a great trip. Happy Anniversary
