try having a look at http://www.ticketmayor.com/venues-tickets/Madison-Square-Garden.php They start New York Liberty vs. Atlanta Dream tickets for 44$. Ticketmayor is generally cheaper than stubhub...
It depends on the venue.Hockey = 18,200Basketball = 19,763Theatre = 5,600Wrestling = 22,292Boxing = 20,789Concerts = 39,522 (unless the artist has a special stage set-up then it could be more or less)