[nilkn]

I don't think salary is necessary at all for this data to be useful. In fact, I think accurately portraying salary differences in different regions is a far harder problem than accurately portraying general COL differences.

In particular, this data can be immensely useful if you live in X with a known salary and you have an offer for a different salary in Y, and you're wondering how it'll stack up. This will answer that question for you.