This cost does not include how much it will cost just to get the next MEC design Warden, Sentinel, and Paladin. Each MEC level you take here will cost an additional $25 and 40 MELD.
Building a brand new MEC costs $25 and 40 MELD. If you so choose, you can build different MECs and choose different options to maximize your adaptability in combat. A soldier is not permanently connected to one MEC. You will be able to build MECs for your soldiers to use.