If it were a typical 3kW single oven then it would use 3kW in an hour, so 3kWh or 3 units of electricity. At the typical Uk rate of ~15p per kWh, then this would be ~45p per hour.
Don't forget the thermostat will be turning the element on/off as necessary thus not full on all the time, so costs less than that in reality.
If it's a 4Kw oven then ~60p per hour, 5kW would be ~75p per hour.
Not sure how you'd measure how much Gas you would use, in comparison, as I'm an electrician not a plumber ;-)