This does not actually appear to be the case from what I can tell. All it does is give you a boost to the percentage you can spend on espionage, for example if you had an espionage ability of 50% and income of 1000bc, and then set your espionage spending to 10%, your total espionage spending would be 100Bc + 50% - 150bc. The main difference is that this 50% is not exactly a bonus - all 150bc gets added to your expenses. (You can hover over the spending on the espionage tab to see this calculation). I'm not sure if this is how it's supposed to work, because this doesn't reall give you much advantage over someone with 0 espionage bonus - they could set their spending to 15% and have the same spending (150bc). The only bonus you get is that you can put more than 25% of your income into spying (25+50% = 37.5%). Do you think this is a bug or how it's supposed to work?