An architect agreed a fee of 5% of the total cost of a project. The contractual sum turned out to be considerably more than the price of the finished project. Is the architect legally obliged to return some of his / her fee?

This would depend on the nuances of the contract, but in most cases it is reasonable to assume that the architect will only be able to claim 5% of the actual construction cost, and be obliged to return any amount in excess of this.