There are a few purposes for the combos I have currently.
The terrain + zone kits combo were to encourage people to try both kits out. I really want everyone to try both and come back with a very good experience.
The other combos are bulk buys of 10-20 kits. These were created to encourage larger orders to really get the HiddenForest brand out there. I believe it is also beneficial for people living outside of NA which saves them a little in shipping.
As for the other combos that are being requested 2x terrain or 2x zones. I am already selling the singles really cheap and I'm not sure how much of a deal I could cut.
TL;DR I have combos to encourage certain buying behavior, but everything is still cheap!