I'd reckon that the Palace of Versailles cost a helluva lot more than Mount Rushmore.

Edit: I haven't been able to turn up good numbers for the original cost of Versailles without spending more time than I have to spare at the moment, but on an ongoing basis the annual budget for Versailles is about €100 million, while the annual budget for Mount Rushmore is about $5 million.

Edit #2: Looks like the Brits spend around £50 million annually for the maintenance of royal palaces, and those are still in active use.