Well, I can't guarantee how espionage funding works but they say you can turn off the spending when you reach advanced and thats what I do. I continue to get stolen techs long after i have turned of the spending so if it runs down it runs down slow.
Now, the stolen techs are great but if you want to talk about the accuracy of the info provided by espionage, it is apparently about as reliable as the real life intelligence that we see in the news. In other words take it with a grain of salt.
Count the ships shown in the diplomacy screen and then compare it to your espionage report, you will see what I mean. I am also pretty sure that your spys are duped by any improvements like spin control etc that distorts military power.