Portable electric oil heater, or portable electric ceramic tower heater? Right now I heat my small shop with an electric oil heater, which just radiates heat. It has a 600W mode, 900W mode, and a 1500W mode. To keep the shop at 55 degrees, I usually have the thermostat half way to thee quarters on 600W mode, and when I'm in there and I want it at 70, I usually start at 1500W mode with the thermostat all the way up, then switch it over to 600W mode. I don't know the exact cost, but I believe this is costing somewhere around $40 a month to run. Part of the problem is that the thermostat isn't based on air temperature, so if it's on 3/4, it would keep it around 55 on a 25 degree day, but if it's 35 degrees outside, the shed will be like 70. So I often use too much heat.
I was considering buying a ceramic tower heater, which includes a programable thermostat based on air temperature, has a fan blowing the air, and oscalates. These are usually 1500W. I don't know the actual efficiency of these things, because it's probably running 24/7, but then again it may distribute air better.