What is the maximum possible amount of earned credits from a playthrough?

A bit more than 700000. Don't know precise number. Doubt anyone does.

Addendum. Discounts are quite worthy in the long run, especially if you ever will be playing new game+. I am not sure if credits would be saved for NG+; though i am very sure weapon upgrades would be. And one can buy weapon upgrades for credits, so at least in this form, credits are transferable to new-game+. You probably need to know that weapon levels from 6th to 10th are only unlocked in single player if one starts new game+, and that the cost of lvl6+ upgrades is quite high - up to few dozens of thousands of credits for level 10 "spectre" sniper rifle (black widow) and such weapons, if i remember correctly. It took someone 4 games played with same character (3 new-game+ ones) to get every weapon to level 10 (except one which is bugged and stays at lvl 7). Person had some DLC/promotion weapons, but not all; he estimated that with every possible weapon it would take him 5 games to upgrade everything he could. So i think that discounts are quite justified, even to the point that player would want to hold with purchases until he gets discounts.