If insulated, you need less BTU to heat up and maintain. If starting from 50 instead of 0, you need even less. When I had a large unheated shop with no insulation, I used a big kerosene torpedo, that was 150 or 200k. When I downsized to a smaller shop, I used a smaller propane torpedo that was 100 or 120k. I liked the torpedoes for the forced hot air to warm up the work area faster. After I insulated, I used a variable BTU propane torpedo that was also a quieter model and would turn down the BTU once warm, like 30 to 60k. Now I have an insulated attached shop and heat full time to 60 plus with a natural gas hanging shop heater that is 50k though 25 would be fine (I did not know initially whether I would heat full time or as needed).
I like being able to go there comfortably any time. I have AC that I run in summer. Took me years to get to this point in life, but I like it. Maybe I spend another $1000 or even $2000 per year on heat and electricity — I don’t know for sure and am fortunate to be able to afford to do so.