@Azita3 If your co-host is only in charge of communications, and not physically attending to the homes, I'd recommend offering a fixed monthly wage. Freelancers working side gigs like this really appreciate having a consistent, dependable source of supplemental income, especially when the assignment requires them to be on call 7 days a week to get those responses out in time. In turn, you retain a trained assistant despite having only a part-time position to offer.
As for the dollar amount: in your case, the position is basically a virtual assistant. That job goes for roughly $18 per hour in California, so for a commitment of up to 1 hour per day, $550-600 per month seems like a fair starting point. But if times come up when you need the co-host to handle things on-site, a commission on each stay she handles would be appropriate.