not really sure about specific calculations but i guess it's tied to the in-game faction score modified by difficulty (probably based on the difficulty level and number opponents, possibly scaled to map size as well) and number of turns it took you to win. so basically everything that increases your faction power score also increases the final score at the end of the game and the trade off for maximizing score is higher base score vs. higher score multiplier (from finishing sooner)

at least that's my understanding how it seems to work.