Tariffs vary but one kilowatt hour of electricity typically costs about 7p. If your dryer is of a type which can be plugged into a 13 Amp socket (rather than requiring a separate electrical circuit), the highest possible power rating it could have would be 3kW. Therefore, the maximium cost of running your dryer will be about 20p per hour. (Yes, I know 7 x 3 = 21, rather than 20, but that 7p per hour figure is only a rough estimate). I doubt that many dryers use much less than 3kW - otherwise they wouldn't produce enough heat - so that figure of 20p per hour is probably the best estimate you'll get unless we're told both the exact power rating of the appliance and details of your electricity tariff.
Chris