As long as it's all clearly spelled out ahead of time, I think organizers can have whatever compensation formula they want. 

Our monthly series pays a per person guarantee -- 1 caller guarantee and up to 4 band guarantees, and performers split equally any attendance-based bonus (if there's extra money after guarantees & expenses.)

When I'm calling, I like to get at least the same compensation as the individual band members.  If a band happens to have 7 musicians, I'm not psyched to split it equally. 

You could make it a $200 performer guarantee, with minimum of $50 for the caller if the caller doesn't travel "with the band" (as callers do with Wild Asparagus and Elixir, for ex.). That way, a caller's pay wouldn't be negatively affected by a chance booking match-up. 

Chrissy
Belfast ME

 caller, musicians, (and sound person) a guarantee of $50 each, up to $150 for the band.  We are considering simply paying $150 for the band, even if there are only 2 musicians. ...  [so] musicians get a minimum of $75 each, but the caller still gets a minimum of only $50. Hardly seems fair. ...