At the Salt Lake Bike Collective we've got a valet coordinator in the summer who works pretty intense shifts, sometimes triple-booked. In the past we've charged $12 an hour for the service, which is what we pay our coordinator. For some groups that's a total dealbreaker, but we've grown to the point where that's what we can sustain, and we'll still subsidize things we really want to encourage a bicycle commute to and grandfathered obligations from when we did it for free.
I have been having meetings with our largest customers (150+ bikes per event) about increasing that rate to $15-20 an hour to cover admin and material costs, which has to be determined on a case-by-case basis: Some orgs balk at the price, others agree no problem. For our largest event, the Twilight Concert Series, we charge $24 an hour for 2 employees and park 400-600 bikes a night, and are considering charging a flat $40 an hour for that event.
My thinking is: encourage and grow the service where you want it to be established by subsidizing rates (farmer's markets, recurring events), then when it's become a substantial volunteer and employee burden increase the rates so you're not paying to do what is essentially a municipal service. Make sure the organizations that you do valet with know that they're getting a reduction ("The valet costs us $20 an hour to run, we only charge our employee's cost @ $12 an hour, and we're willing to do it for free for the first year to see how the community responds to the service at your event") so they're not surprised when you up the rates.
Davey