Late last year there was a survey that came out from HSBC that stated the "average expat adds an extra $21,000 to their annual salary by moving overseas."
In this article from the BBC, there was a quote that reinforced this idea that expat assignments are more lucrative than remaining in your home country:
“One year as an expat could be the financial equivalent of three years back home."
Other details from the survey were that 45% of expats said they got more money for the same job by moving abroad and 28% received a promotion.
The best destinations for the largest increases to salary were Turkey and Indonesia, but China, the US, India and Singapore were very good options too.
While making more money may be a nice benefit, there are still other issues that impact the decision to take an assignment like environment (culture/pollution/etc), safety and security, and work/life balance. Germany actually came out on top in job security and work culture which are also big attractors for expat employees.