Do a bit of research and you should get the price down.
I have a friend who is a type I diabetic with a number of complications (occasional dizzy spells) which rules out being able to drive a car or ride a regular bike.
I was able to source a kit adult tricycle for about AUD$300 and a bike conversion kit from http://dillengerelectricbikes.... for AUD$700 (replace the front hub). There are some shipping costs which I haven't included, assembly took about a day and a half (one day for the trike, 4 hours for the conversion kit). I have come across adult trikes are used by the maintenance crews on industrial sites.
In hindsight given that it was a trike I could of gone sealed lead acid battery conversion kit (AUD$360 - includes electric hub, battery, controller and all the components). Providing that you are moderately competent with hand tools such as a spanner and hex keys you could replicate what I did for AUD$800 / USD$600.