They have quite a few coke vintage sites on the net. Depending on the condition and what the ad is in reference to - it could be worth anything from $10 to over $100.
I was talking to a friend of mine the other day and she collects coca cola memorabilia and I can tell you that it has cost her a small fortune to do so.
I would go to google and do a several searches and put some inquires in the contact section. Also - take a picture of it and upload it. Have you tried putting it on Ebay?
Most importantly, I would wait and see what offer yields the most before letting it go. Hope this helps.