If someone gives me an estimate to carry out an agreed amount of work, then after me agreeing to that level of work and the price, the contractor takes a deposit from me, starts the job, then fails to deliver by the agreed date, then increases the price of the job to almost double the agreed price, then threatens to stop work until I pay the new and what I consider to be an unacceptable price, what level of increase would the law consider to be justified or unreasonable?