sure thing -- but, since "stability" is a rather subjective factor, you will have to decide what kind of weighting to apply to the (purely mathematical) eCPM results. it seems, from the manner in which you've posed the question that you equate "stability" with volume -- which is not un-sound, but not the entire picture, to be sure --
for simplicity's sake, let's assume both your campaigns ran in a 24-hour period -- i would create a table of values --
perhaps 0.1 to 1.0X -- and make this your "Stability Factor" -- the table would be based on the campaign's delivery rate -- for example, a delivery rate of <1,000 imp/24-hours = SF of 0.1 and a delivery rate of >100,000 imp/24-hours = SF of 1.0 -- again, this part is subjctive and you must decide how to apply the weighting -- in your example, this would reslt in an SF-Adjusted eCPM of $4 for the "unstable" campaing and $30 for the "stable" one -- if it were me, i would create a separate column for SF factor, so I could see the SF value separately -- this way I could tell at glance that the $40 eCPM has more to it than meets than the eye -- otherwise you wouldn't know that the "apparent" ecPM of the "unstable" campaign was so low because of the SF factor --
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j-