Priscilla this is a wonderful post, and I applaud you and the expat team for wanting to collect this data. Let me recommend that you create a post and pin it (and close it) so that it is readily available to those looking for cost of living information, and it can be updated only by the expat staff. Longitudinal cost of living data would be very useful.
However, your survey captures only one side of the picture. Depending on your source, you may find claims such as cost of living in Puerto Rico is 80% of the U.S. average, or cost of living in Puerto Rico is 120% of the U.S. average. Both of these claims are correct, but they are measuring cost of living differently. The first is similar to your method -- what is the cost of some hypothetical "basket" of goods and services, and how does it compare to the same "basket" elsewhere? This is fine, as far as it goes, and that basket is cheap relative to other places. As an example, the cost of housing in PR is about 80$ a square foot, while on the mainland it's $100 or so (depending on locality).
The problem though is that salaries are not comparable, and if you assume that the expat will be earning as well as spending off the local economy, then Puerto Rico becomes expensive. My wife is an RN, earning about $45K/year. In Puerto Rico she would earn one-third of that salary, if she could find a job at all. If the price of goods and services is 80% of the mainland average, but salaries are 33% of the mainland average, what is the real cost of living? Economists have a term for this -- purchasing power parity.