Other answers include good estimates, and show that for one printer the electricity costs are not very significant. If you are using many printers and want a concrete answer, then it would be wise to purchase a power monitor (for continuous monitoring) or multimeter with clamp.
With a clamp-on multimeter, you can clamp the meter on to the 3D printer's plug and read the Amps being drawn. Assuming a 120 VAC single phase supply (typical for North America), the power consumption is 120 VAC multiplied by the Amps drawn by the 3D printer (P=VI), which you can read from the multimeter.
The amps drawn by the 3D printer will vary throughout your print, but for a longer print, you should be able to get a good average amps read during a middle print layer. Total energy cost of printing per day would then be:
C = (V*I/1000)*t*E*n
C, Total printer energy costs per day ($/day)
V, AC Voltage (V)
I, Average current draw during print (Amps)
t, time printers are running per day (Hours/day)
E, Energy cost from utility ($/kWh)
n, number of printers