Spies often work for free, based on their ideologies. Agents, on the other hand, usually get paid whatever their normal pay is (aka, rank), plus danger pay and per-diem, where applicable.
Do you have a reputable source for that?
That said, I don't think I've actually seen this distinction made before, and I'd love to hear citations for all the assertions that OP so confidently makes.
Sources:
- Spy museum: Berlin
- Human Intelligence School for the US Amy (in ~07-08)
https://en.wikipedia.org/wiki/By_Way_of_Deception
Interesting book about how the Israelis infiltrated Iraq's nuclear weapons program, and the rivalries between the Mossad and other internal organizations in the 80s.