What factors affect tile purchase cost?

In my experience, the factors which effect purchase cost are:

• Current turn/era (the later the costlier)
• Number of tiles the city already has (the larger the costlier)
• Distance from the city (the further away the costlier)
• Whether the tile is one which is considered for expansion - i.e. has a purple border (if it is, it's about half the price)

For a more complete reference, below is the section from the game configuration file which deals with "acquiring new plots", as it is called there. Any entry with "culture" applies to culture "cost" only, but I'm not quite sure what the entries with "influence" mean; perhaps this applies to both gold and culture cost.

(the section was originally in XML format, but I've changed it to something friendlier)

Cost of acquiring new Plots

PLOT_BASE_COST = 50
PLOT_ADDITIONAL_COST_PER_PLOT = 5
PLOT_COST_APPEARANCE_DIVISOR = 5
CULTURE_COST_FIRST_PLOT = 15
CULTURE_COST_LATER_PLOT_MULTIPLIER = 10
CULTURE_COST_VISIBLE_DIVISOR = 5
CULTURE_PLOT_COST_MOD_MINIMUM = -85
MINOR_CIV_PLOT_CULTURE_COST_MULTIPLIER = 150
MAXIMUM_BUY_PLOT_DISTANCE = 3
MAXIMUM_ACQUIRE_PLOT_DISTANCE = 5
PLOT_INFLUENCE_DISTANCE_MULTIPLIER = 100
PLOT_INFLUENCE_RING_COST = 100
PLOT_INFLUENCE_WATER_COST = 25
PLOT_INFLUENCE_IMPROVEMENT_COST = -5
PLOT_INFLUENCE_ROUTE_COST = 0
PLOT_INFLUENCE_RESOURCE_COST = -105
PLOT_INFLUENCE_NW_COST = -105
PLOT_BUY_RESOURCE_COST = -100
PLOT_BUY_YIELD_COST = 10
PLOT_INFLUENCE_YIELD_POINT_COST = -1
PLOT_INFLUENCE_NO_ADJACENT_OWNED_COST = 1000