Why would you compare SuperNats to a Bertil Roos weekend? A more fair comparison would be a club race. You can do an arrive-and-drive kart race in a competitive field at my home track for about $300, including race entry.
Why would anyone who is not a committed, competitive kart racer show up out of the blue to race SuperNats or a WKA event? SuperNats is the most competitive kart race on US soil. A Bertil Roos weekend is, um, not the most competitive car race.