With Machine, C-level and Betalevel we have worked with a free event with donations/tips policy. Betalevel and C-level have had historically low returns on passing the hat. Machine has had better luck ($200-$300) month with attendee donations, which is attributed to the construction of the cash machine
We have heard tale of other venues getting some income from charging at the door. The going rate seems to be $5-$10.
The free event structure creates what is sometimes refered to as "an economy of attention". As cultureal producers, we believe that attendance from an audience has a certain value. As such the exchange becomes between the performance/lecture/etc being exchanged for the attention of the audience. However, this exchange can be felt to be unequal in either direction. If the audience feels that it is paying mroe attnetion than the event warrents, they will leave. no problem. If the audience feels they are getting more than they deserve for their attention, this inequlaity can be problematic. We suggest constructing some kind of release mechanism for this feeling. The cash machine is a good example.