Maybe they're just rubble when you've just destroyed them, but they become more and more attractive while you pass through the ages. After all, if you look at ruins of something like a modern supermarket, you don't see something special. But these ghost towns in the (mid)west of America are already becoming attractions. Same goes for old industrial buildings (remember that huge factory on one of Pink Floyd's record sleeves?). I'd say two coins per era. Giving a total of 10 coins for ancient ruins. A big kick in finance if you've pillaged two AI or barb cities in the same city radius on useless tiles like desert or tundra in the radius of one of your cities.