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In my mind, 15% seems like a fair figure if they're just supplying the advertisers, going up to 30-35% depending on how much more they handle. Or maybe 40-50% for initial buys, without residuals. (No data, just gut figures here.)
Basically, I see them as outside sales staff. I'd prefer to just work directly with them and encourage them to handle the same clients year after year. I handle all of the ad delivery in-house. They'd handle contacts, creative, etc. I like the guy that runs it, very pro ...and he knows how I like to work.
Just got back... we set up a 25% agency discount, they do all the client contact, creative, BILLING, AND will handle all issues between the client and me, such as placement, ad types & sizes, renewals. Sweet!