I mean, check your bill. Did the price per CCF increase from previous months? Did your transmission/storage/dozen-or-so-other-random-fees increase dramatically? If not, then yes, $180 IS what it costs to heat your particular 600sqft apartment to 68-70 degrees in this cold, regardless of what it seems like it should cost.

Did your usage more than double, according to your bill? If so, you’ve definitely tracked down the problem. Maybe your apartment, small as it is, is cheaply built, poorly insulated, and has shitty windows? Maybe the furnace is old and inefficient? You could always try weatherproofing and/or running an efficient space heater like an electric oil-filled radiator to see if that helps.