How to manipulate the costs?

I don't know if it is possible, but if you want to "cheat" so you can use a trainer and add 10000 gold or more to you. And then you can do every spy mission you want (and so often you want).

You should be able to adjust the cost for spy missions in the editor. Start the editor, click on Scenario then check custom rules. Then go to the rules editor and click on the Diplomats and Spies tab. Have fun.