The answer to this _very broad_ question depends on many factors such as (but not limited to):

 - field: I can't really say if the same position in one field makes more than another but it's very possible. 
 - country: see other answers; North America or Western Europe are waaay to broad to be clumped into a single bucket. (see below for approx figures from Sweden) 
 - university/city: bigger and more expensive cities usually call for larger figures, but in practice you don't get richer on that due to higher cost of living (particularly rent)
 - financing: postdoc but where? ... at a company/university/independent research institute? In certain systems it's also possible to apply for [postdoc grants][1] (in Swedish), I know some people who have gotten grants that include their own salaries. Getting such a grant means your financial burden on the employer is much less.
 - tax: some countries have special, time-bound tax classes for "visiting scientists" or something like that, I have heard that Denmark has a such a policy 

That being said, I have got a figure of [approx 32500 SEK/mon][2] (in Swedish) which corresponds to roughly €45000 per annum (The source is a labor union for university employees).

Hope that helps

  [1]: http://www.vr.se/forskningsfinansiering/bidragsbeslut/internationellpostdok.4.41c4c50b1195b50750780002260.html
  [2]: http://www.saco.se/Yrken-A-O/Larareforskare-vid-universitet-och-hogskola/