Redhat RHEL Server for $60 or RHEL Desktop for $30 for Students and Instructors.

From: Lynn Dixon 
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I just seen this today, and bought a copy myself.
If you have a .edu email address, you can purchase a copy of RHEL Server for
$60 or RHEL Desktop for $30.  Considering RHEL6 Server is $349 normally, its
a pretty good bargain.

http://www.redhat.com/solutions/education/academic/individual/

If you don't like paying for Linux, theres always CentOS, CentOS 6 has been
out a few weeks now :)