The AxiDraw power supply is 9 V, 1.5 A, or 13.5 W, and usually uses less than that, but can also potentially use slightly more accounting for the inefficiency of the power supply. For two, figure maybe 30 W total? For a full month, 24 hours * 30 days = 720 hours, or 21600 watt-hours at 30 W, 21.6 kWh.
Figuring $0.15 (US average) per kWh, I would estimate that two AxiDraws would together use about $3.25 per month in electricity, assuming that you leave the motors powered on all the time. (Use Extensions>AxiDraw Utilities>Disable XY Motors to disable an AxiDraw, and it will use much less energy.)
I don’t think that this is enough to explain $40-$50 difference. On the other hand, if having them connected is (for example) related to your computer not sleeping, that kind of thing could cause a much larger difference in energy use.