I have had to deal with players wanting to send spies in other campaigns. Simplest way is to ask what information the 'spymaster' was seeking, the amount of time and money they wish to devote, who they would send (each had chosen a known NPC so skills were easy). I divided the money spent into the time given before a report was due as a bonus to the NPC's dice roll. If the information desired was basic the roll was easy, specific it was harder, detailed it was extremely costly in time and gold to get a shot at it. The spy was discovered on a fumble and got special information on a critical. Only one player devoted enough thought to answer the question of what information he wanted, spent enough money to get good current information and kept track of the answers he received. Most could not be bothered. The player would give me the 'questions' one session and I would 'answer' in a later session when the spy reported. He called one group of spies Rovers, the traveling merchants, bards, thieves etc and another group the tailors; those that moved into an area or were recruited and reported only when it changed or made a good enough roll to find out something special. The player built up maps of rover routs supporting tailors and was able to create good maps and information of the land being spied on. It was great for fleshing out an area before they went there or getting them to go to an area. When he wanted more he had to come up with a way to get it and then pay for it in game time and money. He even figured out how to get the spies to pay for themselves by selling some information.
In short get the player to do the work, you just give consistent answers based on his notes. Keep the rolls simple and make the player pay for it.
