Depending on the size of the system, yes, it can be.
The emergency backup electric coils are 12kW (give or take). If it runs 12 hours/day in emergency mode, and using 10 cents/kWH, that's $438.30 per month just for the heat.
Guess I just don't know if 12 hours a day (50% runtime) would be normal? And is this only when it's below freezing outside? I remember I was looking at a low-ambient temperature unit a few years ago that said it would work to 0 degrees, but I think that was in combination with electric heat (but not 100% resistive heating).
I'm interested because I have been considering moving away from town, but there is no way I could afford to treble my energy bill.