Are you kidding me? As a business owner I would NEVER enter into a profit sharing arrangement with a webmaster, SEO, or other. Most smart business owners don't - employees get paid a specific wage to do a specific job. How much I make is of no concern to you.
To the original poster, ask for a specific salary based on your region, experience, and how much you want to make - if they can pay you what you want, great. If they want to negotiate go for it but don't settle for less than what you think you deserve. If they can't pay you what you want (or need) then don't take the job. But don't even suggest a profit sharing model, any business owner worth their weight in salt would boot you out the door immediately.