well, i suppose if you have a pump, waterblock, and radiator just sitting around then the system may cost you $15 bucks.
otherwise,
a pump will cost about $20 for a good one
a waterblock will cost about $40 for a good one
that's $60 bucks right off the bat.
then you have to find OR buy a radiator, hoses, clamps, resorvoir, fans, etc. etc. etc.
you can do without some things, but don't believe the hype about setting up a system for $15.