Well, here's some math on that. The battery pack I have in my kit-built electric bicycle has roughly 624 watt-hours in it, and being generous/lazy and not accounting for conversion and charging losses, thus costs about $0.049 to charge from zero to full (which I never do since I don't run it flat) at my current grid rate of $0.0789/kWh. That is, 4.9 cents. Slightly less than a nickel.

It'll propel my ass (along with the rest of me, usually) about 18 miles without pedaling, albeit not any faster than about 25 MPH.