1957 503 cabriolet

I have a friend that is going to sell a 1957 503 cabriolet. It is a very original unrestored car that looks like it just rolled off of the lot. Where do I look to find a value for this car?
Thanks

I saw one in the LA Times want ads a year or two ago for somewhere in the $6k range but didn't act on it. I am looking for one to buy and would be a cash buyer. Please contact me if interested (858-759-7939). San Diego, CA