I'm from Mizoram. In my experience, Speed Post is one of the best couriers for this region and perhaps the cheapest except for sellers who have a tie-up with one of the private couriers at a special rate.
Speed Post has a simple pricing structure which is easy to understand. It is based on distance categories and actual weight. Here's the link:
http://www.indiapost.gov.in/SpeedPost.aspx#Domestic_Tariff
In addition to the price calculated from the chart, they charge 10% service tax plus Re.1 education cess.
For example, if you send a package from Mumbai to Mizoram, it falls under the >2000km category. The rate is Rs.80 for the first 0.5kg, Rs.40/- for every additional 0.5kg.
If the package weighs, say 0.9kg, Speed Post will charge you 80 + 40 + 10% + 1 = Rs.133/-.