If they are getting foggy and hazy from sun damage, it would be worth a try at re-conditioning them. There is a thread on using the 3M Headlight restoration (search) kit-read it. I've become pretty darned good at this. I did my sons' '97 Lincoln Town Car and they were just awful. Had to start with 240 grit to remove the damaged surface and work up to the 500 that the kit starts with. It might save you a heckuva' lot of cash.

I'm just sayin'...

DIY: Headlight Lens Restoration / Polish - bimmerfest - BMW Forums

If you have the time, there is a great pictorial guide to doing this. It was on one of the BMW forums.