Well, if you think about it, retailers usually jack up prices by about 30%. How much does an average Huffy cruiser cost? I dont know, but the cheapest bike I'm aware of, brand new, is Walmart's Next Challenger R at $88. Those motors look stock but hey, whatever you can do to sucker people out of money, hey?
Anyway lets have a look here:
$88 - Next Challenger R
$150 - 66cc HT engine (Type D)
Total = $238 + 30% = $309. People would swarm you like locusts at that price, the stock engine is plenty fast for most people, but lets make it a sweeter deal.
$120 - Arrow reeds
$35 - 19mm Dellorto PHBG clone
$10 - JB Weld crank stuffing and transfer corrections
$25 - Custom exhaust
$35 - Jag clone CDI
~$100 - Hub adapter and chosen sprocket
Free - Engine balance
Free - High rev porting
Free - Head shaving and lapping
Total cost - $563
Total power - Probably 7hp if done properly, maybe more.
+ 30% = $732. But with all that, you could probably get more, considering it'll still have better range and waayyyy more power than any prebuilt electric bike in that price range. But the reason electric bikes arent everywhere? some people just dont like the idea of assisted bicycles, I like to think most simply can't afford it.
So bump the price up to $1000, which is pretty good for a motor with that kind of power. Ditch the hub adapter and go with a 3-speed fixie hub (so you dont have to crank the pedals to start the motor) for $140, then add a shift kit for another $180 for a total of $783 Get a better bike, maybe one with disc brakes, +$112 you're lookin at $5 shy of $900.
$900 + 30% = 1163. There, a decent bike, powerful motor, ease of use, great range and gears, all for the price of some of the cheapest electric bikes out there. Would anyone buy it? probably rare. But for $300, let them swarm.
Related to the post: No, as everyone else here has said, thats way too much, but yet, seems right around what everyone else seems to sell complete bikes for (brand new)