It turns out we are acting on two kinds of "conversion" cost:
1) Intercultural conversion which occurs when you change over a building from one culture to another
2) Intracultural conversion as in att_effect_building_conversion_cost_mod
I think both are overpriced in Vanilla and encourage the use of these variables.
At the very least reduce 1. As it is it is cheaper to sack or burn down a level than to convert a building.
The building_min_conversion_cost in cvariables is actually not a factor in a complex formula. If you reduce the conversion costs of buildings in the range of -65 or so, you will hit this variable which serves as a floor. In my tests, all tier 1 and 2 buildings, regardless of culture, cost this much to change to another version once all conversion costs had dropped low enough.

Steam\steamapps\workshop\content you find your new mods in there

See discussion for news. Does anyone have a better mod-manager loader? I can't seem to get my mod manager to locate this mod in my data directory. I can't upload the new version for you.

Are you saying you see know difference between -2 backend and -150?
I am not an authority on the backend effect as I haven't used it yet in my mods. I know that Vanilla and other major modders give backend POSITIVE values to decrease conversion cost. for that effect. construction_cost_mod uses negative values.
I am looking for feedback from testing. I will give it a try as well later this week. I

Actually I am confused a bit to be honest. I just checked in my mod I only used handicaps and it works. It reduces the cost significantly. example: -2 True att_backend_main_building_conversion_cost_mod faction_to_region_own_unseen -150
. I gave -150 because my buildings are more expensive btw. So what am I missing here if the handicaps are applied and conversion is cheap already?

It seems like you found a way to do this properly. Very soon I will probably have to tweak some in my overhaul :) thank you