The variable that predicts your cost is not ad spend. It is decision volume: how many times a month someone at your agency makes a judgment call, writes an explanation, or absorbs a client anxiety.
So your price sheet needs a second axis. Score every prospective account as low, standard, or high decision volume and attach a fixed dollar amount to each band on top of the fulfillment cost. A dollar amount, not a percentage, because the work is not proportional to spend.
Three costs are almost always missing when we audit a partner pricing model: unbilled account management time, sales cost amortized over expected tenure, and churn drag. Load them in and the margin you thought you had usually drops by half.
A floor is only useful if you can say it out loud without flinching. Ours: nothing enters the book below 45% gross margin after those three costs, and nothing below a fixed monthly minimum regardless of percentage.