300bc a week is fairly cheap in the long run lol.
Don't forget the stealing of tech at Advanced levels when you have spies still placed. There is a benny.
As for strats w/ spies, as Willthemailboy said, a wave of spies is important as the AI won't necessarily be stockpiling spies to combat 10 or so. They will chip away at them however, but as long as you can produce spies as fast or faster than them, it will sort out in the end.
Spies also make for a great economy killer. As long as your economy can deal with long-term spy combat, you can sell some stuff to the AI to empty it's treasury, then pile on the spies until their spending becomes more than their income...target morale and econ tiles (not necessarily farms if you can kill the morale buildings to make them dive in morale, if no morale buildings then axe those farms). Once you see them faltering due to long-term economy crunches they will back off of the spy spending and you'll really have them all good to go for war

Hint, when selling techs pre-war and pre-spy econ crashing...go for planetary improvements that they share the tech tree with you...their social spending will crunch them as well.
Same goes for some high end race you want dethroned (not by you though) while they are at war. If you can clamp their economy, their war won't last too long.