Here is what my tests revealed:
att_effect_building_conversion_cost_mod faction_to_region_own works if negative
att_effect_building_conversion_cost_mod region_to_region_own not tested
att_backend_main_building_conversion_cost_mod works only on buildings of other cultures.
Surprise: Positve 100 will make it about 2x as expensive to convert the buildilng. 200 will make it about 3-4 x as expensive. If you make it negative, you can make the conversion cost of other cultures very small.

building_min_conversion_cost at 111 333 555 all have no impact on conversion costs within the same culture. I wasn't able to test it on buildings of other cultures, but I think it works like the backend_main effect.