What fuel source options do you have (e.g., electric, natural gas, etc) and how much do they cost? How well insulated and sealed is this space? Do you need AC too?

Electric space heaters and such are 100% efficient in terms of converting electric into BTUs of heat, but have a Coefficient of Performance (COP) of only 1.0. A mini split has a COP of about 3.0 and will therefore use only one third the electricty as electric space heating. However, for such a small space and likely very small BTU load, I doubt the ROI would be positive if you could find a small enough system.

If you go with an electric space heater, I would think having a thermostat would go a long way to reduce operational cost and improve comfort. This link might be helpful:

Tiny House Energy Efficient Space Heaters

Depending on the intended use for the room this may not be appropriate, but a $30-40 electric blanket on the floor works great for providing comfort while hanging out on the floor, even when the room temp is 60F or cooler. In Japan they sell heated floor pads that work even better than an electric blanket designed for beds- you may have to fold the blanket in half, getting twice the heat under you as when laying flat if it's actually cold in the room.

A small radiator type space heater is less of a fire hazard than some of the other types. A 1500W version with a 600W element and a 900W element is about right, and can be had for ~$50, sometimes less. Most of the time just running the 600W element would be enough, but for bringing the room up to temp faster or for keeping the room comfortable when its really cold out. It's slightly more comfortable to use just the 600W or the 900W for maintaining the room temperature, since it keeps the radiator in a tighter temperature range, with less cycling. Radiator types are also more comfortable than anything using a fan, which makes noise and creates a wind chill effect.

It's possible to do both for less than $100.

Most Efficient Portable Heater


All electric heaters are 100% efficient insofar as 1 kwh of electricity will produce 1 kwh of heat.

Any perceived efficiency will be derived from setting a thermostat or timer to ensure that it's only on when needed so get one with a timer and thermostat.

But you should also know that how you use the heater can determine if it'll be energy efficient or not. Also, what are prices like where you live? You should use that to determine how much you'lll spend daily or monthly

You don't need to spend a lot when purchasing a heater. In most cases, a $20 heater is just as efficient as a $200 or even $2000 one

Most Efficient Portable Heater


All electric heaters are 100% efficient insofar as 1 kwh of electricity will produce 1 kwh of heat.

Any perceived efficiency will be derived from setting a thermostat or timer to ensure that it's only on when needed so get one with a timer and thermostat.

But you should also know that how you use the heater can determine if it'll be energy efficient or not. Also, what are prices like where you live? You should use that to determine how much you'lll spend daily or monthly

You don't need to spend a lot when purchasing a heater. In most cases, a $20 heater is just as efficient as a $200 or even $2000 one

Efficiency is the same for all electric heaters, but the comfort & safety factors vary considerably.

For the sub $100 budget I'm still voting for comfort with an electric blanket + oil-filled radiator solution. It shouldn't be hard to do both for about $75-80, or just the 1500W radiator for $40-60.