It doesn't necessarily work that way in this bidness. Most agencies and I bid based on a flat project fee that takes (normally) two rounds of revisions at each development stage into account. But in broader assignments that will run over several months or generate ongoing work, a monthly retainer is the preferred method. I do that with a few clients. Some months you get clobbered, other months you're getting money for nothin'. But over time it balances out.

This client merited a retainer, but needed to keep the number low to get approval. So I negotiated a monthly base based on a certain number of hours, plus an hourly rate that would kick in if the monthly hours exceeded the base by more than 10%.

Lucky me, both of my client contacts were totally unfamiliar with this kind of work and couldn't get their heads around the review and approval process. Instead of vetting the work through all the decision makers in their company and aggregating the comments in one pass as I begged them to do repeatedly they continually gave me feedback via the drip method. Which virtually tripled the base hours. Which caused a new CFO to try to break the contract which was pretty silly because there was only one month to go before it ended. They paid me in full for the final month and I gave them the entire project history on a few DVDs and bid them farewell.

Five months later, I sent them a duplicate certificate for the Addy Award I won for their branding! laugh