cheapest cpu fan?

I just found out that im having family over and their staying for a month or so my parents called and told me so i had no choice. anyway. does anyone no the cheapest a cpu fan will come? im doing a pc grow box.

Radio Shack has basic fans for like $5-$10. I've seem fans at the bigger electronics stores too. Amazon.com has a bunch for real cheap too. Good friend of mine just bought a few of them for his PC that move some serious air for their size and they were like under $15 a pop.

sweet thanks.