Just built one of my own. With the very large electric motor I put in it, it's faster off-the-line than most sports bikes.
Features a boat tail, smooth wheel covers, air deflectors, a belly pan, max sidewall tires, economy instrumentation, and electric assist (1kwh battery).
The trike I picked up on Craigslist for $600. I probably have a similar amount into the electronics - I could've set it up for as little as $250 or so, but it would top out at 19mph rather than 51mph. The body is coroplast, zip ties, duct tape, plus some pipe hangars, threaded rods, bolts, and aluminum L.
EDIT: It also has a "trunk" which I can stand in, and can take probably 2-3 large bags of grocers.
EDIT2: I currently have parts on order to add turn signals, brake, and running lights to it.
