Where are you going to be? There are some hotels not far from the marina in Rodney Bay, non-waterfront and so cheaper than the ones on the beach. My wife and I stayed a night on one of them a few years ago, waiting to fly out after having our boat hauled. It was the Bay Garden hotel
, if memory serves; found it simply by Googling *hotels Rodney Bay St Lucia*. It was nice, clean not too expensive and had a decent restaurant--maybe US$80 per night. Try Googling *hotels Gros Ilet St Lucia* also. TripAdvisor.com is a source of info, too.
Depending on when you'll be there, there could be a minimum stay requirement. Try looking for "guest houses" or "B&Bs", as well.