The CHEAPEST solution is to open the door from your house and allow the Heat from your house go into the room (didn't your parents ever yell a you to close a door, or was that just me?). Opening the door is the same as connecting it to the HVAC, if you want it to heat faster then you put a fan in and blow the heat into the room.
The original qustion is a trick question. To calulate how much heat is needed you need to measure heat loss; which is the heat lost over time and can't be determined with just a single measurement of outside vs. inside temperature. To properly do it I would measure it overnight so that you can exclude soar radiation effects, sounding fun so far? The reason it was 40 was the heat loss from your house warming it up, so that's a good sign that it can retain some heat, you just need ~40% more heat in the space.
You should consider adding insulation if there isn't much. Heat loss is a function of insulation, so a poorly insulated space will cost more and be harder to bring up to 70 (since its losing heat as fast as you put it in the room). I can guarantee that if you had enough insulation the room could be 70F from the houses heat loss.
For myself, electric heat costs 9 times more then natural gas heat for the equivalent amount of heating. 24 hours of space heaters costs the same as 9 days of HVAC. You say you won't use it much, but the 9:1 ratio is pretty huge.