About $350... But seriously, the $500 scope will almost certianly have better quality lenses, superior coatings, probably be more durable, probably be easier to sell used, and most likely will not let you down on a hunt. At least that is what we are supposed to be convinced of in order for us to fork over the extra $350. I don't know that I will ever own a $500 scope, at least in 2005 dollars. I have been pretty impressed with the $200 to $400 range myself. I just don't believe that a 100% increase in cost justifies a 7% improvement in performance (or less). But then I haven't ever used a $500 or more scope under "ideal $500 scope conditions". Like during a rainy dusk with still legal hunting hours when that nice buck steps out at long but reasonable range and you have to be certain that he is carrying at least 3 points on one side (to make him legal). Maybe with the $150 scope you can't be sure, but with that "Scheist Concorde" $500 3x9 you can be. At least if you believe their marketing, and many swear by it.
I still say to buy the best optics you can afford, and if you can afford more than $150, don't be afraid to spend more. Chances are you will get what you pay for, for good or ill.