I've visited BJJ schools all over North America, plus a couple in Europe, and I've never paid anything unless I'm there for a few days. Like clinzy, I always offer and (with two exceptions out of probably 30 academies) am always turned down, so I try to do something else if I can.
Come join us at MECCA if you find your way to downtown Toronto, ON. http://www.meccamma.ca. We'd never charge a visiting BJJer - we're big fans of being welcoming and offering open training. We have visitors from all over and it makes us better.