I agree with Roundabout. Next time get a friend who'll pay for the site 100%. Then you do the work and split the profits after he gets his money back 50/50. That's what I did. I think there's a similar concept in real estate.
You guys both invested money to purchase the domain.
You will invest effort to monetize the domain.
You both should be compensated for your investments. Track the hours you will spend working on the site, and when the money comes in, be honest and show your friend your hours and how they break down by category; SEO, web site development, etc. Don't forget hosting, and calculate taxes in there too.