If we were comparing the cost of living between different countries, we could use "The Big Mac Index."
Basically, the theory behind it is, the exact same product is delivered in a very similar environment, or it's a direct "like for like" if a take-away, so one can judge exchange rates, cost of living, economy, salaries etc.
The trouble here is, we are trying to compare two different provinces in Thailand. That said, the price of a Big Mac may be different on Phuket, compared to Chiang Mai. I don't know. I don't eat that rubbish.
We need to have a product that is similar. I think a Starbucks coffee would be the same as a Big Mac. Ok for comparing countries, maybe not so great for comparing provinces, but who knows, they could vary in price.
Beer is another good product for comparison, because a Singha beer on Phuket is the same Singha beer in Chiang Mai, the problem is comparing the environment in which it is delivered, which effects the cost. Eg. a nice air conditioned bar with the sport on, versus sitting at a pile of bricks in an outdoor beer bar.
There's no use comparing street food, as there are too many variables.
Perhaps some members can thing of a product that would be a direct comparison between Phuket and Chiang Mai.
What about market foods? Eg. coconuts.