> In the case of the Boston cleaner, that seems to indicate the living costs can also drive salary. If nobody applies to work somewhere, the wages either go up or the job goes unfilled.

When one job salary doesn't cover living costs, you are more likely forced to have two or three, not a raise in your current one. It also solves problem of unfilled jobs. Because there are cases when job is only economically feasible if performed in urban area with high population density, so you can't really change your location.