Originally Posted by 2old

Difficult to locate one (except maybe Amazon) in your price range from what I've encountered, but look at electricbikereview; they may have tested something. If this were my goal, it would be DIY with a (hopefully) Craigslist bike or something in the $300 - $500 range and a hub (probably front) kit. DW has lots of experience with "folders"; maybe he can help.