In order to encourage countries to build a decent spy network, why not offer the ability to investigate successful spy ops. It should offer a chance, at the expense of a few turns, to investigate an op to fing out who did it. Or maybe give a list of a few countries that could be responsible. That way countries are not deterred by huge expenses of maintaining a big spy network, and gives a counter to spying. The higher your spal, the more effective the investigation.