When you need to cook something small but don't need a huge oven to cook that food then using a toaster oven will save you money on electricity.
The toaster oven uses around 1,000 to 1,500 watts of electricity while on and cooking while a large electric oven consumes around 3,000 to 5,000 watts of electricity while on.
So if you're just baking something small and don't need the space of an oven and you have an electric oven then the toaster oven would be cheaper to use.
However if you have a gas oven then the gas oven would be cheaper to use than the toaster oven.
Cooking with natural gas is much more energy efficient and cheaper than cooking or baking with electricity.
But if you have an electric oven and need to cook something small then the toaster oven would be cheaper and save you money.