Howdy.
Let's face it, using spies to gain "high" levels of intel on other races is just not worth the effort. All you really gain is the ability to look at their planets, something you can do in the spy placement screen regardless of your intel level (although it's a smaller display, the improvements are still instantly recognisable if you're familiar with the game). I've been going through entire games neglecting espionage completely, because a) the information gains are too small, and b) sabotaging alien improvements are extremely costly since they tend to "nullify" a new agent within four weeks tops, usually within two.
The game boasts that at "HIGH" espionage levels, you know more about them than they do, and yet you can't even see what their tax rate is. I suggest some real gains: where their sliders are, what their income and expenditures are, what they're researching, where their ships are going, which planets they're focusing on if busy attacking, what their short-term, mid-term and long-term plans are, even what their general strategy is. If espionage can help one determine where their weaknesses are, THEN I'd say it's pretty damn useful. Looking at their planets is fun, but not worth 300bc per week.
J.