If you want a more accurate answer you'll need to find out what the power rating is for the dehumidifier you have or are looking at. and then it's really not hard to work it out yourself.
You pay for electricity in KW/h (kilowatts used per hour) so check your bill for what you pay for this (we'll assume 15p)... so to run a 1KW item for one hour costs 15p
Then just look at what the item is rated at in watts (1000 watts is one kilowatt) and it's very simple to work it out.
500watts is 1/2 a kilowatt... so 15/2=7.5p per hour.
250watts is 1/4 a kilowatt... so 15/4=3.25p per hour
And so on.