The expense is going to depend on many factors. If you go with the electric radiant heat you need to look at the electrical requirements for the coils when you determine how much electricity it consumes you can multiply that by the amount you pay per kilowatt hour for electricity to see how much it will cost to operate monthly.and then determine.
If you go with a liquid filled pipe then you need to determine the cost of the heater to heat the fluid and the circulation pump.
One option is to use solar to heat the fluid then a small circulation pump powered also by solar power. If the pump is mounted low so it doesn't have to do anything but slowly circulate warm fluid, and not pump it way up, then you will not need to worry about monthly expense just the initial cost of the fluid setup and the solar heat and solar power set up.
