I try not to worry too much about prices, that will drive you crazy.
I have been to all of he islands you list except st.Lucia in the last few years.
My favorite place to provision was LeMarin in the south of Martinique.
Leader Price has a dinghy
dock that you can bring your grocery cart to.
The quality of the French food (and wine) is fantastic. There is also a Carefour supermarket within walking distance.
The market in St.Georges Grenada should not be missed. It is huge, colourfull, vibrant. Excellent local fruit and vegatables, not expensive. You can tie your dinghy
to the wall in the carenage and walk through the tunnel to get there.
market here is also very inexpensive, but it is hit or miss as far as selection, they have whatever came off the boats, selection changes hourly.
The women in the stalls will prepare the fish
as you like, amazing to watch them work
The market in Bequiia is nice, but the vendors mostly come by ferry
from St.Vincent so it is more expensive.