I think this still covers the basics, although you'll have to factor in the increase in fuel price since last year...
http://www.pprune.org/private-flying...ml#post3652894
You might also be able to offset your group running costs if you make the aircraft available for hire and commercial training, but this can take a bit of management effort and obviously reduces the availability.
From a competiton perspective, trying to have more than two pilots share an aircraft at the same level in a competition becomes problematic - partly because as a competitor you don't want to be rushing to about to get the pilots swapped over in a timely manner, and also from the organisation point of view juggling the flying order can become difficult.