how do u figures this out..is it relatively easy?

It is easy.

http://wiki.earthempires.com/index.php/Relations

This page is unclear and I would edit it to but it is locked. The part where it talks about consuming your NW should say that is the per turn expenses. For example a defensive ally costs .002*your_nw each turn.

that wiki is really helpful for returning players who just need quick numbers

that's why it was made.