unfortunately the code to determine which plot is the next one to acquire is one of the worst pieces of code i've ever seen. it seems to work most of the time, so i'm loath to touch it but here are some of the constants which go into it. if you want to experiment, go ahead and change them in the database.
base cost (determined by pathfinder, aggregated from city plot)
INFLUENCE_RIVER_COST()
INFLUENCE_HILL_COST()
INFLUENCE_MOUNTAIN_COST()
pTerrain->getInfluenceCost()
pFeature->getInfluenceCost()
additional per-plot constants added afterwards:
iPLOT_INFLUENCE_IMPROVEMENT_COST
iPLOT_INFLUENCE_RING_COST
iPLOT_INFLUENCE_ROUTE_COST
iPLOT_INFLUENCE_NW_COST
iPLOT_INFLUENCE_YIELD_POINT_COST
iPLOT_INFLUENCE_NO_ADJACENT_OWNED_COST
least cost wins.