It is quite simple to work out.
Electricity is sold in units of kWh, kilowatt hours. ie one thousand watts running for one hour equals one unit. If the device does not consume 1000 W you simply divide 1000 by the wattage of the device to give you how many hours you need to run it for to consume one unit. So a 500W device would run for 2 hours, a 250W device for 4 hours per unit.
Once you have determined how many hours it takes the device to consume one unit you then divide 24 by that number to work out how many units it uses per day. So a 250W device uses 6 units per day (1000/250 = 4, 24/4 = 6).
You then multiply that figure by how much you are charged per unit to work out how much it costs per day. Say you are charged 10p per unit that means our theoretical 250W device costs 60p per day to run. And then you can derive the cost per week, month, year or whatever.