Update: Detailed video tutorial! http://youtu.be/raBfRj1SW2o
This is a companion Thing to my Micro-Adjustable Bare Wire Z End Stop ( http://www.thingiverse.com/thing:23811 ) -- this is actually the way all of my end stops have looked since January.
This is about as inexpensive as you can get with your endstops, and yet they've worked flawlessly and predictably for months on end. Each endstop is fashioned from a paperclip, a short piece of copper wire, some electrical tape, and a twisted pair from an old network cable.
They are NO (normally open) switches as opposed to NC (normally closed) so you'll need to configure your firmware accordingly, and you don't get the fail-safe you would otherwise get with NC switches -- but the trade off here is in the simplicity of the mechanical action, arguably lower risk of mechanical failure, and the lower vitamin count.
1) Find a paperclip from under the couch cushion or the back of the desk drawer, and an obsolete network cable or telephone cord.
2) Cut the paper clip and shape it into two "L" shapes, and crimp, solder, or tightly wrap the short ends of the L's with the network cable.
3) Poke the long ends through a strip of electrical tape about 3-4mm apart, and tape the assembly down securely onto the end stop holder, making sure the wires won't touch.
4) Tape a short piece of copper wire to the extruder body, print bottom plate, or X end so that it will complete the circuit when it hits the paper clips.
5) Marvel at the simplicity and reliability, and then proceed to upgrade the Z axis end stop to my Micro-Adjustable Bare Wire Z End Stop ( http://www.thingiverse.com/thing:23811 ).
I haven't had to do this yet, but you'll probably need to clean the wires at some point to ensure a good connection. A quick scrape with fine sandpaper will probably do the trick. I've been told that paper clips don't conduct electricity. I'm not really sure how to respond to that other than to say that these work great -- perhaps they're coated with a conductor.
Legal Disclaimer: The "One-Penny" bit was a guess. You should be able to find someone to give you a paper clip for a penny. If not, I'll gladly send you one if you cover shipping and handling.