I also see the 20" amazon motor kits from China with controller/display for under $275, as low as $250 depending on display, You get the IT4 cable harness and waterproof connectors all around, plus a controller box. I've never bought them or seen one, but it seems like a nice package, if these are bafang motors. .
All my 20" bikes are powered by Akeima hubmotors, mainly the Q100H. Those cost me about $130 shipped. Ive spent $80-100 buying spokes/rim. Another $100-120 to buy a controller, display, and a case. Amazon is a better deal.
I doubt I'll ever build another conversion though, as long as I can buy a BBS02B for under $375 from pswpower. The TSDZ2B is $100 less, but won't knock your socks off like the BBS02,