500K is the current market price which is approximately a hefty $200K mark up. But there are people who are prepared to pay any amount to get the latest and greatest.
Used 430 spiders are trading at $100-125K mark-ups. Coupes for about $25-50k mark-up.
Reasonableness is a relative concept. The guys buying a 599 at market price should be multi-millionaires at least. So paying 200K more would be a mild irritant to them if at all.
5-7 years from now, you can buy a used 599 for $150K. The guys buying at market will take a nice bath then.