Your electric bill is calculated in KiloWatt per Hours (kWh). Take the wattage and divide by 1000 to get the KiloWatts, then multiply this by the number of hours that it is running to get the total kWh. Finally take that number and multiply it by the rate that your utility is charging you.
wattage / 1000 = kilowatt (kW)
kilowatt x duration = kilowatt hour (kWh)
kWh x utility rate = cost to run
If I were to do a brisket for 16hours out where I live, here is what it would look like
MES 30 - 800W /1000 = .8kWh x 16hrs = 12.8 x .16362 = $2.094
MES 40 - 1200W = 1.2 kWh x 16hrs = 19.2 x .16362 = $3.141
So a MES 40 would cost me $1.40 more than the MES 30 to run over 16 hrs or .0875 per hour.
Of course this is if the element is running at the highest possible setting the entire time. The element would not be on the highest setting or on all the time.