I noticed that there are some significant variation in the salary of post-docs in Germany. The post-doc salary could be given in TVL13, TVL14, TVL15, and even TVL15U scales. This means that the net income could change from 2,000-4,000 Euros. I have currently an offer to start my post-doc with TVL-13 (Stufe 4) which gives a net income of 2,900 Euro. Is this something reasonable? What factors are important in determining the post-doc salary? and how could I change them?
Postdoc salary (like for any non-tenured academic position) in Germany is fairly rigidly structured. The salary is always according to the federal wage agreement ("Tarifvertrag der Länder", TV-L) and is determined by
Only the second point actually depends on the applicant, the rest is tied to the position and needs to be fixed before it is even announced (e.g., as a 75% TV-L E13 position) and is therefore non-negotiable. The second point is somewhat negotiable by trying to get them to recognize previous postdoc positions with similar responsibility as time on the job (although that is an administrative matter, and the possible influence of the academic staff on the administration varies wildly...)
Which pay scale and in particular percentage is reasonable is very discipline-specific. If it's a full-time position with no responsibilities (such as running critical experiments for everyone else), it seems not bad because Stufe 4 is actually comparatively high (E13 only has five grades).
From my experience working 4 years in Germany, I can tell you that you got a reasonable deal. I don't know about those 2900, but at least in the Hamburg region that would be around 2500 Euros net.
You can check the values of 2015 (and beginning of 2016) here.
The TV13 is the standard position for researchers (Wissenschaftlicher Mitarbeiter), which is basically the same as the PhD students, but a higher "Stufe" because of your years of experience. If you manage to get some special responsibility or lead a small group of students ("group leader"), it could be possible to upgrade it to TV14, but that depends on your group leader.