I currently have no information about the salary of a post-doc here, but I can tell you, that depending on the luck with fuding the same Ph.D. student get 560 €/month after taxes ( I was really considering getting a second job like.. "what else can I do instead of biology?, mmm, nothing well enough probably, so.. shop assistant in Lidl?"...) and a year later 1200 €/month after taxes
Situation is rather complicated by taxes, because full-time students up to 26 don't pay them, also full-time students can be funded from certain parts of a grants that are considered a gratuity, which is also not taxed. When I had a break in the studies, that meant my salary went down by 120 €/month, because I had to tax everything I got and the amount didn't change.
Also Ph.D. students in the first 4 years (which is considered to be a normal duration of Ph.D.) get around 240-320 €/month scholarship (not taxed) from university, which is generally the main reason why everyone wants to employ a Ph.D. student. Because they can pay them only half the salary from their resources.
Of course that in the capital the numbers are bit different, but I rather don't think much about it, after all money is not my important motivation.
If the question was also about if by working on contract you have a fixed salary, then no. Usually you got a one-yer contract each year, and the salary depends on the actual grant funding available. You never know what will be next year.