This on its own isn't enough to refute OP, since it could very well be that the equivalent cost of one of these other historic buildings is even higher. (e.g. cost of skilled slave labor vs equivalent labor in modern markets, cost of materials, etc)

EDIT: Hmm, even the payer's power could be factored in. A king may be willing to spend 1/10th of his empire's wealth on an extravagant palace, but the same would likely not be approved in a democracy.