The prices in Puerto Rico grocery stores versus the prices in other US grocery stores, such as the nearest mainland city, Miami, can be readily compared online.
For a 1.5 liter bottle of water:
Puerto Rico: $1.58
Miami: $1.73
Most grocery prices are about the same, although milk is more expensive in Puerto Rico.
Another source, suggested by Adam, shows that the food component of the cost of living index is 4% lower for San Juan compare to Miami.