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Hi here are some sites to use as reference for the estimated cost of the plastering -
Here is a link to get some estimates for your proposed job in your area -
Material cost can be found here - http://www.buildingmaterials.co.uk/Plaster-Walls.html
Well I hope that helps get a idea of the cost involved and get a more reasonable quote that can work with... Let me know how it goes and hey all the best.