.10 cents per amp. each video card uses 2.4 amps. So so 4.8 just on the video cards and maybe anoher 1amp on the PC itself. So figure 5.8 amps.
5.8 amps. so 5.8 x .10 = .58 cents per day. =17.40 cents per month.
Electricity use is charged in watts (includes the voltage the amps are pushed at), not amps. You'd be using maybe 650W, or 0.65kWatts per hour. That's about 468kWhour per month, at 13 cents a watt that would be $60.84 per month. Probably a conservative guess on the high side.