The rates at Simpson Bay Marina and Lagoon Marina will run 2-3K$ per month in season; I know because I tried to negotiate good rates last season. I know 2 people very well, permanent liveaboards who are anchored next to Explorer Island, who would be reliable for guardiennage. Once you are there, the Soggy Dollar Bar (during daily happy hour) is where you'll find candidates. It is a pretty regular group of people - probably less trustworthy when it comes to non-boaters, but a solid group when it comes to fellow boaters!
Jolly isn't a great place to
anchor out, but if you really want
Antigua then I have some reliable contacts in Falmouth who would watch a boat for a period during the season. You can always send me a pm here if you need further information.