To highlight the examples above....
My brother makes 6 figures as a nurse in NYC (Manhattan specifically). His measley eff apt ran $3K/mo! Then there is the NYS income tax, the NYC income tax, 8.25% sales tax, yadda yadda. I earn $22.05/hr + diffs. (I am still NG status.) My mortgage on a 2K sqft 4 br 2 ba house is $1300. I pay no state tax. Sales tax is 8%. Gas is currently $3.47/gal (not sure what it is in NY right now, but I guarantee it is higher! ) Because of the housing market crash, you could buy my house for $70K less than I paid right now and the hiring of RNs is up here. If you have exp.....a specific hospital in this town pays $25 + diffs. We have a few good neighborhoods and 2 HS's in the top 10 in the country (they are magnet schools so your kids would have to have great grades to get in.) There are problem neighborhoods and schools so you would have to avoid them.
The crux....I make far less than my brother but have a better purchasing power! Also, in many smaller areas, you may have census issues that cost you hours or PTO usage. Here...we never have census issues! In fact, my hospital allows OT if they have need and many nurses do it to pay off debt....including paying down their mortgages before retiring!
With 'average' salary sites out there....I wouldn't give much credibility to them. They are usually higher than what you can expect to earn. Cost of living sites are fairly accurate. Cost of Living Calculator, Salary Comparison - CityRating.com this site says that if you earn $50K in Jacksonville, FL....you would need to earn $156K+ to have the same standard of living in NYC!