Over $50 increase on a $500 return? None, especially since I tend to bump returns up 5-10% annually.
Now if a $300 return doubled to $600, I anticipate I would lose that client to H&R, Liberty, or some EA that charges a fraction of a CPA. But, given $300 is basically my floor (with only a couple of exceptions), that is a very simple return and I would not anticipate any need to charge more, anyway.