This is not necessarily the case.
In some countries years of experience increase the salary - but many countries have standardized government grants for post-docs that are independent of years spent in employment (i.e. from PhD to faculty You'll be earning the same on the same grant).

In Europe in most places You will probably earn around 2000 Euros per month after taxes as a post-doc (in scientific disciplines at least, no experience in the humanities), varying widely depending on who is paying You.
It depends on the country, whether the post-doc grant is taxable or not, the local tax rate, and it may include or not include a number of things (health insurance, unemployment/social security, etc.).
The lower end is around 1500 after tax, the highest starting I've seen in Europe outside industry is the Marie-Curie grant including mobility allowance (on the order of 2700-2800 depending on local taxation).
i.e. It's about 2000±25% after tax, with some outliers.

(The UK is a bit of pain in this respect - living costs are higher, but the salary is if anything less.)


This nature article from last year may be of some use:
http://www.nature.com/naturejobs/2009/090205/full/nj7230-750a.html

Also - here are this year's "subsistence rates" from EMBO grants (i.e. post-doctoral salaries depending on country), usually they are a decent indicator of what is +- "normal" for a science post-doc locally after tax, though they tend to be on the high end of the spectrum (especially for middle-income countries - You won't get anywhere near this much from a local grant in the likes of Hungary or Slovenia):

http://www.embo.org/documents/ratesnational2010.pdf