We pay for all of our own personal consumables (strings, leads, straps etc) and put 10% of the fees aside to pay for replacing\maintaining the rest of the gear.
Once a year we sit down and take a dispassionate look at what we've got equipment wise and decide if we need to plan for an upgrade.
If we do, then we do it.
If we don't then we leave a suitable amount in the kitty and split the surplus out between ourselves.