WordPress MU

I am trying to design how could be implemented a pay-per-more-space feature on my site.
Currently there is a space allocation that is defined for all users in admin options. Can I give a specific user more space than the others, and how ?

I don't think there is any space allocation per specific user. All standard sizes, would love a feature like that though.

I'd love to see this feature to be implemented directly onto the software itself. Maybe a plugin for this would suffice.