Red Hat, S.u.S.E., Caldera, Debian, and Slackware are the "big five" distributions, listed more or less in order of current popularity. All were priced in the $30-50 range, although the most recent release of Red Hat seems to have escalated to about $75.
BTW, "S.u.S.E." is a German company; the initials stand for "Software und System Entwicklung."