Hi - In my experience, assessing a cost on incoming revenue is by far the most common approach to fiscal sponsor cost recovery. I'd estimate 90%+ of professional sponsors use this approach.
Within that bucket, I've seen sponsors that:
• have a flat fee percentage no matter the amount revenue (like Jennifer described at SEE)
• have a graduated percentage that starts higher and goes lower as revenue increases (like Josh described at Mission Edge)
• have a graduated percentage that starts lower and goes higher as revenue increases
And there are other ways that it can be done. The most common I've seen are:
• a percentage cost charged against *expenses* (rather than revenue)
• a flat annual/monthly/quarterly fee (this is rare, but it happens)
Of course when it comes to negotiating rates in grant agreements (government or otherwise), that can open up another whole can of worms...