As with its Age of Empires II version and similar to Blockade, Spies have a variable cost that depends on how many units that non-defeated enemies have, but with no minimum or maximum limit.
In a team game, only the players who researched the upgrade can see the enemy's line of sight, it is not shared with teammates.
Except if the starting age is set to Imperial/Post-Imperial Age, Spies is unavailable in Deathmatch as players cannot advance into the Imperial Age.