I probably woudn't advise using a headphone output to record with.
A headphone jack is amplified and your computer sound input is line level.
All you'll get is a whole bunch of distortion.

Probably the cheapest way is just get a mic off eBay. You can easily get an SM58 for less than $100.
If you want cheaper than that, go the Radio Shack route for about $30.
If that's still too much money, go rummage around in a garage sale or something and look for an old cassette deck microphone with a 1/8" plug.
You can probably get one for .25 cents.