Not really sure what you are wanting.
This perhaps?
https://science.howstuffworks.com/i...ons/how-to-make-potato-powered-light-bulb.htm although you are right this would cost you 2p and cost of a few nails (I'm assuming you would have grown your own potato).
You could take a car alternator and attach it to a bike on a stand and use the DC current to charge a battery. You'll get a free old bike off facebook. An alternator might cost you a few quid from someone scrapping a car but if you pedal hard enough you'll soon pay that off.
Or if you are feeling lazy you could build a small wind generator from the alternator and a bike wheel.
Alternatively you could fit PV solar panels to your house it would pay for itself in about 5-10 years and the remaining 10 years of service would provide completely free electric.