[JohnLBevan]

Surely the free market covers this? The contract specifies estimated and maximum costs for the project, and presumably includes a definition of the scope / requirements to be covered and delivery date. If the client thinks you're charging for work you're not doing then presumably they think you're too expensive, so should the two parties should agree to terminate the contract and the client should go and find someone else to build it. If they can't find a better rate then so what if you're charging them to spend a few hours a day on Facebook; the fact you're still able to deliver your side makes that irrelevant.

The only issue is if the maximum cost is far beyond the estimate and there's no justification of why that may be the case beforehand. If there is good justification before hand you should be able to let the customer know if that risk's been realised and report on it immediately, giving evidence to show that this risk became a reality.