You didn't seem to take power consumption into account when calculating the running costs. Your own figures show the Xerox uses 50W in standby whereas the HP uses a much more svelte 11W. I'd like to see the difference in cost if both machines were left on standby for a day or even a week.
And that's without mentioning the consumption when running - ranging from 253W to 480W. Depending on how much you print, that could really add up.