I wanted a 500W rear geared hub motor, because we already had a factory EG bike with that power system. Earlier this summer, I found a 36V 500W geared wheel kit from ebay seller ebikeling for $199 with free shipping. I bought a 36V10AH battery from ebay seller diyebike for $208 plus $80 shipping from China that came in a week. The kit was pretty much plug and play and has been run about 200 miles. That's about 8 charge cycles and nothing has gone wrong yet, but I was probably unwise to buy a no name battery.
The geared drives we have are pretty good on small hills, and with pedaling I can can get my kit bike up a hill I used to walk. My top speed is around 20 mph with throttle.
With doodads like new tires, new front wheel, speedometer, brake upgrade, luggage rack, and lights, I have about $650 in my old steel frame bike. I can buy a new bike like an XB 300-LI Trail Runner for $749 shipped, although it is less power at 300W and 24 volts. Cheaper to buy? Probably.
Still, I am having fun with it. Maybe another three weeks of riding. Winter is coming so we won't be riding any bikes for six months.