Bread machine with maximum power draw of 430 Watts.   The average program time, which includes mixing and rising time, before baking,  is 3 1/4 hours.  If during this amount of time the maximum power usage such as that required to  bake at the highest temperature for the entire 3 1/4 hours, the power used would be about 1,400 watts used for the whole time.  This is 1.4 kilowatt-hours.   It would cost  us what a Kilowatt- hour costs on our electricity bill.

But, the machine does not always draw that much power, since much of the time the machine is kneading , and incubating the loaf-in-progress at a merely warm temperature for getting the yeast to grow, so really the number of kilowatt hours used is probably less than 1 kilowatt-hour, which may cost around $0.10.  Therefore,  the electricity alone costs much less than two pounds of ready made bread.  

There is the equivalent of 33.4 Kilowatt-hours in one gallon of gas.  So the equivalent amount of gasoline to power the bread machine for three hours is roughly  1 kilowatt hour  divided by 33.4 kilowatt hours, or 3% of a gallon, or the equivalent of driving a little less than a mile.

Here is another way to look at  the CO2 production of a person  The average person puts about 2 lbs of CO2 into the air in a day.   This is 1/10 th the amount of CO2 produced by using one gallon of gas.  

A person uses about the energy of a 100 watt light bulb, which is about 2.4 Kilowatt hours per day.  This 2.4 kilowatt hours divided by 33.4 kilowatt hours per gallon of gas is equivalent to about 1.4 pounds of CO2 per day.  Close enough to the above estimate of 2 pounds CO2 per day mentioned above.

Anyway, a bread machine contributes less CO2 to the air per loaf than a person contributes in a day.