Mine was 300 used but I got it from a kid who made it to ride to school/work at Georgia Tech. Aerospace engineering major. He rode it daily for three years and I've had it for a year and change but I only ride it once a month or so. Other than the donor bike, it's just cheap chinese components. He did rework the controller to remove the limiters (top end and phased power delivery) so I can't let anyone else use it as they'll hurt themselves.
Would probably cost around 500 in materials and 10 hours or so if you put it together yourself. You don't have to spend huge bucks on one unless you want to. The cart was 100 from amazon - could not find a used one.