Admission free/ donations accepted

My friends and I are going to see Shakespeare in the Park next weekend (Hamlet, here in L.A.). Parking and admission are free, donations accepted. I've never been to a production where it was admission free, donations accepted- what is a good donation amount? Should our donation cover as if we would have paid for parking too?

I'm not totally lame, I promise :) it's just that usually I attend events where it's "donation of $XX.XX suggested" and so I know where to start at least.