I can easily now verify that every type of Bishop is cheaper than Mages. I own both a lukless mage and bishop, and have played avg luk and low luk versions of both (with exception to lowluk FP mage).
The bishop will almost always be cheaper. Gen's MP costs about half of Blizz/Meteor. If I used Gen at 120, I could pull off 3-5 Gens depending on how often I get hit. Using Blizzard, I can only pull of 2 at max (assuming both skills are lvl 1). My lukbishop/mage friends said they could pull roughly 2-4 Gens at 120, and 1-2 Blizzards (if they got hit, it was ALWAYS 1). It took our Bishop nothing but a clean shine wand to 100% 2 hit skeles at 138, and we didn't have Gen 30. When equipped, it is already coming close to 1 hitting skeles at 157. My luk bishop friend could 2 hit at 14x, but had to be equipped pretty well. He also could pull 1 less Gen+hit than us.
For the reasonable and realistic bishop, lukless is MUCH cheaper for the damage/results produced. For the perfect ideal bishop, very lowluk is best (this assumes you Maker and Chaos almost perfectly)
For Arch Mages, there is no debate. Lukless is best. Even if you can only afford lvl 70 ele wands, the massive damage boost will outshine any high lvl luk wand. The boost from MP is a HUGE help with the insane pot costs. Until Nexon releases very high lvl and high INT ele wands with luk, lukless Ams will usually overpower their luk counterparts.