The cost of running say a 9000 - 10000BTU unit can be about the same as running a 3KWatt fire for a week but it does depend on the units efficiency. The BTU rating also gives a good indication of its running costs. A 10,000 BTU machine costs about 10 to 15p an hour to run, depending on the energy rate you are charged. So for a weekly cost you need to multiply that by how many hours you wish to run the unit for. Make sure you buy one with the best energy rating though, the ratings are A to D just like refrigerators, with A being the best