I think the cost depends on the size of the city you're trying to colonise, and also how many units are in your army.
There are a few entries in
campaign_variables_tables that control resettlement, and one that controls the base cost for all factions -
occupation_decision_colonise_cost. If you want to change it for a specific faction, one way to do it is by making a new faction trait that changes the cost for that faction. I've actually done this in my
faction traits mod (I think it works

), if you want to look at how I did that.