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The basic reason behind cost spike is your plumber may have to cancel other his appointments to perform your emergency work which indeed cost him money.
You should also aware of distance between you and plumber and time taken to reach at your door step. As it can happen that your plumber is already in work and emergency situation arises at your house, so he cannot reach as fast as other emergency services reach. Required time may depend of his current location and type of work he is doing at that instance. If you have contract with good companies then they can provide you estimated time of arrival of plumber at your house. So when you call them for emergency service make sure you ask time before contracting plumbing service.
Above two things you should know while hiring an emergency plumber to show up at your door step. This things will keep you aware of time required in such emergency cases and will give you known knowledge of money required. This can also help to keep your mind calm and think reasonably then being frustrated which will lead to more frustration due to situation.