^ that is a .... weird question, i guess? i not so sure how to answer that question. That somehow depend on your party and play styles. Take Rk for an example, they need to almost continuously spam DB, and maintain their current hp to maximize DB's damage. If you are the type that love to scream "SP PLZ", then it might be cheaper. But for some people, they dont really like to bother or totally depend on the sorc as a sp battery, so they might need to spend some extra zeny in ygg, blue, straw, or the currently most famous consumable : ales.
But i can say that maybe gene is the most expensive to level(?). Since you might need to spend extra zeny on those coats and bomb. True to be told tho, i did partied with a gene who only spam CC and didnt bother to do anything else, so meh?
So all in all, dont bother with which class that the cheapest to level. You are already lvl 141+ by that time. You should at least accept the fact that thing will get harder, and just play the class that you enjoy.
as for which is the cheapest in term of gears, you can go to every class sections, find a few famous build (or guides) and try to calculate their price for the gears. I am too lazy to do that stuff. As far as i know, if you want to survive in bio, you will need standard reduction set. So that is the baseline for the cheapest gear i think.