You will find all sorts of financial models out there, but which one is best for you depends on a few factors.
An organization with a large bank account can perhaps run in the red some percentage of the time. Larry Jennings, when setting up the finances for the Thursday NEFFA Contra Dance in Concord, MA, estimated that we should lose money about 1/3 of the time. The practical implication is that we have a higher guarantee than dances that need to be in the black all the time.
We also allocate a different guarantee per player depending on the number of performers. (Fewer perfs. => more per person).
We also kick in some time/gas money for those coming from a "distance".
Annoyingly, bur probably fairly, we have two "bonus" bands: The first band pays out a fraction (depending on the group size) of the excess beyond a certain gate figure. The second band kicks in at a higher level and essentially pays the performers any excess (this recognizes that super-attractive performers are likely the cause for a much-above-average attendance, so they should be rewarded.
Whatever you decide to do, try and be as fair as possible.
Good luck!