Not acceptable - It's a paid gig . If they want to pass the hat and say something humourous / lighthearted " we need enough for the bus fare home or " , then the observer may or may not elect to put something in the hat.
Otherwise I would rebuke any advance which quantified the amount they thought they deserved. You wouldn't accept a the described demand in a restaurant / bar - why should you in this case .
When Busking a smile and lighthearted quip doesn't go amiss when putting the "bite" on.