Grenada sits among the least expensive islands in the Caribbean. and there are plenty of local markets where you can buy fruit and vegetables quite cheap. There are some restaurants that obviously target tourists and the prices are easily 3 or 4 times more expensive than where the locals eat.
Accommodation could eat your travel budget if you stay at the big resorts. Fortunately, there are some cheaper options available. A couple of budget guesthouses in St George and some Cocoa Farms in the north of the main island are the best choices for those traveling on a budget.
Transport is very reasonable. There are two options: taxis or minivans. The first ones are expensive and mainly for tourists while the second ones are cheap and used by the locals. There is one main road going around the big island so most of the minivans do the same route. You only have to stay by the side of the road and flag down anyone going in the desired direction. The fares are between 1 and 5 EC, depending on how far you are going. The last (and free) option is obviously hitch-hiking!
I arrived to Saint Lucia with the idea on my head that budget travel was going to be difficult here. The island is a popular travel destination for cruise ships or sailboats, but not at all for backpackers. Although, it's the same situation in most of the islands in this part of the Caribbean.
The good news is the travel costs in Saint Lucia can be reduced to a minimum much easier than in other countries of the Caribbean. Budget guesthouses can be found for as little as 18-20 USD in Castries, there are a few well-priced and well-stocked supermarkets and transport in minivans is quite reasonable. Not too bad if you are backpacking on a budget.
Situated among the French Antilles, with Martinique to the south and Guadeloupe to the north, the island of Dominica is much more budget travel friendly than its neighbours.
Life is simpler here. There are hardly any resorts and the tourists who visit the country are after hiking or diving opportunities rather than beach-combing. The geography of the island is quite different to the other places in the area. The terrain is pretty rugged and wild making the transportation to run slower than normal (even for Caribbean standards!). Prices in general are quite affordable. There are plenty of markets and camping opportunities are widely available.
Saint Vincent and The Grenadines
St Vincent and The Grenadines are really made for sailors with their own boat. For the average traveller, you are restricted to a number of islands and the limited options available in terms of accommodation and food. The prices depend on which island you are at. St Vincent is by far the cheapest, followed by Union and possibly Bequia. After these 3, the options for accommodation and food are so limited that normally results in a considerable increase of the prices. Fortunately, it is possible to travel through The Grenadines on small local boats. It's cheap. You will only need time and patience.