tough to say.. something like $30-40/day sounds about what I'd expect for a bunch of places. There are often distinct advantages to riding a motorized bicycle.
Just parked my bike over a weekend in NYC. $28 and I rode it in, parked it in a corner out of sight on the lower floor of a garage with a car elevator. It was fine. A car would have been a different story, methinks. Not sure how comfortable I'd be leaving my bike parked in a parking lot in NJ...