Average package offered to Mtech vary from department to department and this variation is quite large.
As I am from IITR, let me share the placement scenario here.
CSE students get the highest package starting from 10LPA to 30+, average being 15–18LPA followed by Electronics and Electrical students. In Electronics, package offered is different for different specializations. Package for VLSI major is quite high compared to others. For VLSI, it is about 16 LPA whereas for other specializations, it is about 11–12LPA. (Find detailed ECE placement statistics at IITR for placement season 2016–17: Neha Singh (नेहा सिंह)’s answer to What is the starting package after doing an M.Tech at IITs?)
Electrical students also get good placements, though I cannot tell the exact statistics.
Branches like mechanical and civil gets lower package compared to the above. There average is around 7–8 LPA. Keep in mind that this is about the average and not the highest.
Branches like Chemical, Biotechnology or Biochemical do not have very good placement statistics. They are offered quite high packages sometimes but we cannot be sure about the average. One of my friend at IITG (Biotech) was offered 20LPA. One of the reason is, students in these branches are more inclined towards research than placements.