fair pricing of bandwidth

[Lloyd Kvam]

My office is in a building where several businesses share an Internet
connection and simply split the expense.  There are no bandwidth/packet
shaping controls.

I expect we will upgrade to a 10 mega-bit connection with a
lower-committed level of service (possibly 3 mb?).  I'll install
bandwidth monitoring and control.  Each user will get a minimum /
maximum bandwidth allocation.

(Finally the question)
Do any of you know of a fair algorithm for allocating costs with this
kind of model?  My inclination is to simply allocate the cost based on
the share of the minimum service levels.
        
        min0 / (min0 + ... + minN) = share of connection cost for user 0
        
That's easy to describe and hopefully fair enough, but I'd welcome
suggestions for a better approach or a pointer to any papers on the
subject.  

I suspect that a fair system is not linear and that the cost of doubling
the minimum bandwidth should be less than double the original cost.

-- 
Lloyd Kvam
Venix Corp