My thread here is a good 1-stop-shop for answers on espionage.
https://forums.galciv2.com/316263
In short, the +espionage skill only (as far as I've learned thus far) increases how much you can spend on training spies. So for instance, the Iconians with thier +100 spend double what the training slider's % denotes. That slider is a % of your raw income. That said, the Iconians can spend 50% of their raw income on training spies if they so desire while most other races are limited to 25%.
