[How to make your own Skype phone out of a old useless cordless phone.](http://www.jarnaker.com/siemensskype/default.htm) This sort of amateur tinkering is lots of fun! Occasionally, we use the same methods in Skype development and testing

Cost $0.
Material used: one wireless phone, and one 3.5mm to 3.5mm plug (cut in half).
Tools used: screwdriver, knife, soldering iron.

image from jarnaker.com

Disclaimer: this probably can and will void your phone warranty. Neither Skype nor the maintainer of the linked page are responsible for any consequences - you're on your own.