Base Cost (Diplomat / Spy Mission Properties)
Determines the base cost of performing the selected mission. Mission costs are calculated using a variety of factors:

Mission | Cost Formula
Establish Embassy | BaseCost TargetCitySize
Investigate City | BaseCost * TargetCitySize
Steal Tech | (BaseCost * Target Player’s Number of Techs * Tech Rate) / 100
Steal World Map | BaseCost * ( ( Map Width Map Height ) / 2 )
Plant Spy | BaseCost
Steal Plans | BaseCost * Target Player’s Number of Units
Initiate Propaganda | BaseCost * Target City Size ( BaseCost * Target City Size * Culture Ratio) ( ( BaseCost * Target City Size) / 2 * Gold Ratio )
Sabotage Production | BaseCost * Target City’s Number of Shields
Expose Mole | BaseCost

After the cost formula is calculated, the cost is then further modified, taking into consideration (where applicable) the distance of the target, the target city’s population level, the ratio of citizens between the initiating civ and the target civ, and the safety level modifier (which the user selects when attempting a spy mission)