How to export all the data of a domain name from domain mapping using sub domain


I am going to use wpmu of some of by clients where they can access all the plugins and themes from my wpmu system.

i am going to use sub domain option and they will point there domain to that sub domain and will use domain mapping for that.

now my question is once they start promoting the domain name and everything runs fine and now if i have to export all there data like post, comments, tags , themes, plugins ,etc . you can say everything which belongs to there domain name then how can i do that ?

how can i export all the things and setup a separate hosting account for them and export there blog to there hosting account.

looking for a solution for this.