business question

Me and a friend run a 'community' site,
as a member you have access to to functionality like chat and forums, and you have your own personalized page with a guestbook ,photoalbum,diary etc..

our site uses PHP and mySQL
Now a company has asked us to run a copy of our site on another domain, basically their idea is to drive traffic to this site and make money by letting advertisers buy ads and banners.

we would be responsible for maintaining this site and for this we would get 50% of the income of the site ..

Has anyone been involved in a project like this? or do you have any ideas what to think of when writing a contract for this project?

(the site is swedish and will only have a swedish audience)