I found this calculator for electric underfloor heating costs
Electric Underfloor Heating Running Cost Calculator | ThermoSphere
I can’t tell how accurate it is, it seemed strange that a bathroom was cheaper than a living room & dining room to heat, our bathroom is the warmest room in the house.
That said it predicted it would cost £2.34 for a 6 hour period to heat my workshop classing it as a dining room & £2.17 if it was a bathroom. Now I appreciate I wouldn’t want the workshop as hot as domestic rooms but chances are I would need to heat it longer than the 6 hour period each day.
With those kind of figures for a month I’d be looking at a higher price to heat the workshop than I pay for gas to heat the workshop & a 3 bedroom house.