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 Players Number of Techs * Tech Rate) / 100
Steal World Map BaseCost * ( ( Map Width Map Height ) / 2 )
Plant Spy BaseCost
Steal Plans BaseCost * Target Players 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 Citys 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 citys 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)