You probably have an older split system than newer heat pumps. We heat about 1200sqft of a century old row house at 68 and the bill is about $150-200, including cooking and other usage.

The bill looks right if your system is not up to date. And if you are a side, top, or corner unit your bill will also be higher. I suppose you either rent or own a condo so changing the external unit would not be an easy option.