How much of a difference is there between the stock steering damper and a aftermarket one?

 

Is the stock one you are referring to adjustable? If not, aftermarket ones are and that is a major difference. The stock ones 'usually' have a full range, vs a set range where at the extremes it cuts off, this shouldn't be a big deal for street riding though.
From my experience, any damper is better than no damper, and an aftermarket one is better than a stock one. But, I don't know if it's $500 better.