I got my first bills and I started seasons on my game in late fall I turned on my thermostat to warmer costs more and my bill was $26,000!! haha (note to self set therm at cooler cost less!!)
According to the coding, the thermostat is supposed to increase bills by 25% or decrease them by 25% depending on which setting you use. Maybe the large bills are a bug or maybe something else is going on, such as having a garden with a lot of harvestables on the plants or in inventories, since that adds to the value of the house and therefore bills.