Pat, I think you and I are from the 'old school' thinking on space heaters. I remember mom and dad using them in unheated rooms over the years in some of the barn-like house we lived in growing up and they drove the bill up markedly, mom said.
Dh and I bought one of those space heaters from Aldi a year or so ago, the one that looks like a small, iron stove (several ladies on another forum had bought them at the same time) and they're much more efficient than the old ones. We run ours all the while we're in the living room in the evenings, plus if the girls are home during the day, they'll turn it on. The rest of the house stays pretty cool, since the thermostat is near the space heater, but we're toasty. I haven't noticed any rise in our bill; in fact, our bill has gone down, due to several factors, but one reason I'm convinced of is the space heater.
I'm not a mathematician either, but I think it's helped rather than harmed our power bills...Liz