I assume you keep your greenhouse heated throughout the winter, Tee Gee. What sort of heating do you have?
You can see my heating system in the first photo!
The bench the buckets are sitting on is my " Hot bed" which is a thermostatically controlled soil heating cable covered in sand. I have not switched this on yet, that won't happen to around January/February time.
When it is on the go it is set to give a bottom heat of around 65°F (18°C)
Below the bench is a 3 kw "Fan Heater" this is thermostatically controlled as well and is set to keep the greenhouse at around 38-40°F (4°C) which i find is enough to keep my greenhouse "Frost free"
If needs be I can use the fan in summer to circulate cold/cooler air around the greenhouse although I never really use it!
I also have a heat sensor in both greenhouses which sends signals to the monitors in the house which I keep an eye on regularly.
The heated greenhouse usually takes care of itself but I like to know what is happening in the "cold greenhouse" so that if it gets particularly cold i can go and throw some fleece over the plants.
All in all I find it quite a convenient set up.
In terms of running costs I have no idea nor do I want to. I see my gardening as a hobby and most hobbies involve some costs and this one is no different!