Toaster ovens do save electricity when compared to using a full size electric oven.
A full sized electric oven can use 3,000 to 5,000 watts of electricity when on and baking food but a toaster oven uses only 1,000 to 1,500 watts of electricity per hour depending on the setting.
If you're just cooking something small then using the toaster oven would be cheaper but if you need to cook something large or more quantities of food then the electric full size oven will be cheaper and more efficient.
Most electric ovens use 50 cents for baking food which is pretty cheap.
A toaster oven may only use 10 cents to 20 cents for cooking food.
The actual cost of using a full size electric oven or toaster oven depends on your electricity rates but the average KWH cost is 10 cents per KWH.
But that can vary from location to location.
Cooking something in a toaster oven for 30 minutes or less may only cost 1 to 2 cents or less.