If you fill a habitat with a dissident faction, it most definitely will cost that one influence per month. Kind of a moot point, but still. I would argue that the mineral cost and the upkeep costs of the structures on the habitat would be a reasonable enough upkeep to give a border range similar to a size-12 planet. Especially since as far as I know, a system exudes only the larges available border range from the system (IE colonizing multiple planets in a system doesn't increase your border range any more than the largest planet colonized).