I get this comment a lot. I agree that it makes sense to compare also local salaries to see which city is the cheapest one to live in as an employee.
However, Expatistan is not about finding cheap cities to work in. It's about comparing cost-of-living, independently of your income. Expats are usually not linked to average local salaries in any meaningful way, anyway.
While actual salary might not be interesting, how much taxes will affect your gross income is. Tax rules are often very convulted. If you collected gross salary and received salary after taxes numbers, that'd be a great resource. I've moved abroad and all you get usually is vague statements about tax pressure (ie. asking a range of people you get wildly different answers). Often this is related to your income level in non-obvious ways as well. A table for each city with mapping from gross income -> income after tax would be very helpful.
However, Expatistan is not about finding cheap cities to work in. It's about comparing cost-of-living, independently of your income. Expats are usually not linked to average local salaries in any meaningful way, anyway.
I explain it much better in the website: http://www.expatistan.com/faq#average-salary