Calculating tariffs

Hey there, i’m a 4th year university student and i am trying to create a network design tool which helps calculate how much it would cost to implment a network depending on the tariff chosen. the information that i have is a graph and the required capacity between the links and i have a matrix of the demands put on the links between the nodes. i was wondering how to calculate the total network cost for the network topology created and how to calculate the cost using the deamnds on the network using the tariff’s offered from each company. i know this may sound easy but i am finding it hard to get my head around it and my lecturer wont help me!! any help would be much appreciated, thanks.