The Colossus was built apparently from the material left by the Macedonians after they abandoned an unsuccessful seige of Rhodes- the Rhodians built it as a thank offering- so destroying it on purpose really shouldn't yeild anything more than the scrap value of the materials involved. This is in fact sorta what happened after the muslim conquest- I forgot the names involved but after the pieces had been lying around for a long time they were carted off by a merchant and sold for scrap.
Some wonders, though, should be worth more, such as Delphi because of all the treasure there. And some, like Stonehenge, would yeild you nothing if you destroyed them but cost a lot to rebuild- in labour if in nothing else.
Is it possible to attach tourist income to the wonders? I know it sounds a little silly but there was tourism of a kind, or in some cases religous pilgramage that generated revenue for the owners. I know that it is outside the scope of the game but the Amphitryonic Council for Delphi and the Shrine at Eleusis and so on played serious and complicated roles in the history of the classical period.
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