https://www.youtube.com/watch?v=kVAZIDFMRXYPretty cheap.
Just plug it to a charger and charge a batterie. When its full use it to mine then when the battery get low, switch back to grid.
You will save some $ (really low) but hey if you do bicycle every day why not save some $ at the same time.

This is what I am going to be doing. But making my own alternator and have a battery bank to store the electricity for later. You always have the grid for when you run out as well.
I think it's a good way to get in shape and stay in shape.
It's not terribly expensive. I have most of what I need. A bike, resin, etc.
The magnets are the most expensive. About $200 to get some bulky n52 grade neodymium magnets for greater power generation and then $60 for some copper wire for the coils.
The design I'm going off of produces about 1000 Watts an hour when on a wind turbine. So I'll see what it produces when you have a geared bike hooked up to it moving faster.
I can also hook up some blades to turn it into a wind turbine when not riding.
It can be hooked directly to the grid too and get some money off of your monthly bill. This way it has more uses to it.