I don't have the exact number (I had only just signed up as a publisher about a month before they closed it -- maybe someone else can pop in with exact numbers) but I remember it being really low, around 5-7%.

It was low enough that it wasn't really a factor when determining what to price your ad-free pages at (most people went with 1-2 cents per load) -- most people just priced at whatever average your ads were pulling in currently without worrying about the cut, since it was a fraction of a cent.