Eh, employers are the ones with much better information in this case. They are the ones who'll know if x% of prospective employees decline an offer or even interview due to the salary offered which will give them at least some idea of the market rate. I've been in the situation where we could not get any new employees due to the rate we offered, and when I suggested we increase the rate I was told flat out by a manager that "Software Engineers should be fine with this amount, we will not hire for over it"