A cheap SMD Surface Mount Device (a little chip without legs that goes on a circuit board) is a 1/10,000th of one cent resistor.[link]
I think a 2 or 3 conductor on off switch SMD could be made, that is similarly about 1/10,000 to 1/1000 of 1 cent at the active SMD component. The great part is you
just move your finger to the colored dot and the thing turns on. move it onto the dot again, it turns off. simple. If you feel like it the dot can glow but that's not necessary.
I think that if you have a single flash memory cell, and load it up with electrons at the factory so that it has a two centuries of them, (they leak out at a predictible rate) and attach it to a transistor, such that the transisitor switches on or off, when a higher than leakage current electron exits the flash memory and says "hi" to it. Then you get a button, or rather a nonmoving touchpad you can press as many times as you like, for two centuries with no moving parts.
Niftily, if it ever pressed the flash memory cell also happens to refill with electrons so it always lasts two centuries from the last button press (IC feature size on-chip circuitry).
Technology: well there's flash memory on wikipedia, and then there's me saying flash memory is sort of like putting electrons in a box, and they have a minute likeliness of leaving the box. Flash drives work that way. The electrons are more likely to leave the box if you, a warm person, and an electrically charged person touch the box.
So the little box the electrons are in has to conduct your body warmth. Perhaps even though it is 1/10,000 of a cent you can get away with chemical vapor depositing a few-atoms layer thick of highly warmth conductive, abrasion resisting diamond on it.
If you are going for the fact that you have electrical charge, and you want to send a ripple of charge variation through the little box when you touch it, then you would use a transparent conductive coating on the box. These, like tin oxide, are well known at display technologies.
There is another one that I like better though, it is an transparent electrically conductive polymer(plastic) called PEDOT. Perhaps you simply do anything from thinly coat to the electron box with tin oxide to embed it in a giant "pushbutton" size dolloop PEDOT. With PEDOT (or metal) there's a big nub you can touch. Anyway, you touching the conductor causes change in the box, unlike flash memory the box, like a weak walled flash drive, is really sensitive to the slightest wiggle, the transistor senses the changes in the slightest wiggle and the lights turn on.
Now, what is it good for? If you read about Plaid conductors, RAIC (redundant array of independent conductors), then the idea that you can use a CPU to find out what's what, or do a sample all the little wires and best "3+ out of 5" (0r 40 out of 1600) defines what the wire is and does.
A redundant array of Indepenedent switches (RAIS) makes it so any human attempt to get a button to work, is likely to work. Instead of corrosion or dust, or shorting, or mechanical-off-flexion, or water, or broken springs making buttons not work, any partial surviving remnant of a RAIS works as an on-off switch. This makes many kinds of machines, all over the world more reliable.
Can I use RAIS at turnable "digital gradient" knobs. Sure, and the advantage is that the component is very break-resistant. Just have an arc of little contacts, each attached to its own weak-quantum-walled RAIS flash drive cell, or perhaps each of the contacts is actually a # microarray of 9 or 125 micronubs. If any of the 9-125 nubs survices to sense the knob, you can still use that knob setting.
If you imagine a halftone arc, you can see how a smooth-soft turn continuous volume control could work with RAIS pads.
Again, part of the idea, is that if you make things that last for centuries, the interaction panels and controls should last. And, they should be really cheap. That way you can get say a tree fertilizer distributor or outdoor irrigation control electronic hub/network object that lasts centuries.
Also, it is just really cheap for RAIS touch switches on things like disposable electric lollipops, electric syringes, and even touch programmable RFID tags. The 1/10,000 cent SMD resistor is kind of a guide to how cheap to make them.