In that case, I still recommend turning on the main thermostat but setting it very low so it only kicks on during extremely low temps as this can help with comfort without much cost at all. The space heater would then do the job in the room you care about to save costs.

One other thing that I haven’t seen mentioned elsewhere and I forgot myself is humidity. While a heater isn’t going to change the humidity like an air conditioner that is cooling does, the fan in the unit can even out the humidity in the house and humidity can have a big impact on how the temp feels. Depending on the size of the room and how much air gets circulated, your presence could have a significant impact on the humidity levels.

I would recommend that whether you run the central at a low temp or not, running the fan for a bit once in a while would be a huge benefit to the home. You don’t even need to turn on the heat for that, just the blower. This will help to reduce any damage from condensation throughout the house, and you don’t need to open your room to get the benefit. Within the room you are using it might be a good idea to check the humidity levels and look up what the best humidity is for the temp you are aiming for. While that might mean getting a humidifier or dehumidifier if the room is way off, that will most likely be a lot less expensive than the heating cost and you might even find a slightly lower temp comfortable and save more than you spent on adjusting the humidity.