Why is this in Toxic Discussions? It seems like a pretty valid Technical topic to me, or at worst, General.
If it hadn't come up on the View New Posts page I would not have seen it.
Anyway, if you want really cheap ways to do it, you should be perusing my Electricle project blog.
I actually *did* do the switch-controlled assist, before I rebuilt a smoked PWM controller off a dead scooter.
The light switch would be more "efficient" than the controller, as far as power loss goes, but it is not possible to do that using a brushless motor, only with a regular brushed DC motor. Note that it doesn't have to be a *permanent magnet* DC motor, only a brushed one, so you can use a vaccum cleaner or weedeater or drill motor as well, or any other series or shunt wired wound-field motor, as well as the usual PMDC types.
And you don't have to buy batteries at all. Just go find people throwing out blown up server-sized UPS backups; they'll have some good-sized batteries in there, often (like mine) 12V 18Ah, and you might get 4 of them, or (like mine) even 8! Since 6 of mine were still good, that's 2 36V 18Ah sets of batteries for nothing. No, they won't last more than a few months, maybe half a year per set, at my usages, probably, but if they cost nothing, what does it matter?
AJ, the reason some of us use wierd stuff for our bikes is because we can't afford any better than whatever we can scrounge.
Some of us do it because we like the challenge.
Some of us do it just so we can say we did it, and watch the goggly-eyes around us.
Some of us do it for *all* those reasons.
As far as the cheapest goes, well, the only cost I have in mine is the welding wire to put the frame together, and the cutting and grinding wheels to cut and shape pieces of frame/etc. Plus a few dollars (literally) on some thrift store items used for many parts for different projects (thus splitting the cost between them)
Everything else came from Freecycle, friends, garbage, alleys, roadside finds, stuff I already had laying around for years unused or broken, and so on.
I bought most of the LEDs used in some of the lighting, but even *that* came off a previous bike, and so was not originally spent on this one (this one started out using just some Honda scooter lighting, including the turn signals and flasher that are still being used on it now).
(none of this says I wouldn't *want* some new parts, but not having any money to spend on things can make you *very* good at figuring out how to do this without using any!)