I've just Googled 'dehumidifier watts' to try to find an estimate for the power consumption of a dehumidifier. They seem to vary widely so it's hard to give an accurate answer but lets assume that a typical figure might be around a third of a kilowatt (i.e. 333W). That means that a third of a kilowatt hour is used every hour, which works out to 8 kilowatt-hours per day.
A typical price for 1 kilowatt-hour of electricity, including VAT is just over 10p, so that works out at about 85p per day.
A small dehumidifier, with a lower power rating, might cost 50p or less per day to run, whereas some of the larger models use around twice the power of my example, giving a figure approaching �2 per day.
If you've already got a dehumidifier (or you're thinking of buying one), take a look at the power rating (in watts) on the label on the back. Remember that my estimate of 85p per day is based upon 333W and scale your figure proportionately.
Chris