My husband performs at ren faires as part of a five person act and he says a dollar tip is totally fine. You can, of course, tip as high if you like if you especially like the show. It's also not unusual to offer a performer a larger tip prior to a show accompanied by a request, like adding a particular song into their set list or asking for a shout-out for someone's birthday or whatnot.
Also, don't let performers guilt you into tipping if you don't actually want to do so. Many performers will make it sound like tips are their only pay, but most of them will have a contract with a guaranteed paycheck, especially if they're a prominent enough act to be on an actual stage with their name on a performance schedule.