With good quality outer lenses (no yellowing or serious scratches/chips) and good seals (not rotted), $500 on the upper end. Bad lenses and rotted seals, $350-$400 (low end).
This is my buying model and I stick to it when buying headlights. This is also a good run-down of actual completed eBay sales.
Most sets I buy end up in the average of about $425.
If they have good "clear" corner trims, add $50.