As the prior commenter noted:
"I guess it it depends on how much work they have to as a co-host. Are they effectively managing the property for you, from setting prices, to cleaning, to responding to and welcoming guests?
if so then it's a lot of work plus you are dependent on them for your great reviews. In this instance they would expect to receive a higher share of each payout (or a fixed fee per month)."
Our co-host does all of the above, including cleaning and general maintenance, and we pay him a straight 40%, which is comparable to other property manement companies in the area which charge 40% PLUS cleaning fees. We do charge guests a one-time cleaning fee, but we keep that and it offsets what we pay our co-host.
Hope that helps.