My suggestion here is to set a resourcecost on sabbotage. Like 20 000 of each resource per sent sabotage and the cost of the spies. Couse as it is nuild a megadef city and barbmuch form start and you will be able to send say 20 sabbotages every 6 hour. The chance to cause som dmg is high and 15 hits. That is 15 lvl and on higher lvls that is 8h+ building a lvl. So you are never really given a chance to defend against this force. So there for the increase cost of resources of it. might even be more expensive or cheaper.