I think I remembered reading that if the first Civil War's losses were extrapolated to our present day population, deaths from a similar civil war today would be over 9 million.
Taken as a percentage of population, I think they'd be similar....remembering that the Finnish affray was blessedly short in duration. Most authorities set 25 Jan 1918 as the start of the conflict, though the first armed battles happened a week previously. The final Red-occupied stronghold at Viipuri fell on April 28-29 and mopping up continued until May 7, though there were many, many executions after that date.
With a total casualty figure of around 36,640, out of a population of some 3 million, they managed a casualty figure of about 1 per 1000.
Given the U.S. Year 2000 Census population figure of just under 300 million, that'd extrapolate here to more than a quarter million dead... but given a probable longer duration, and improved technologies for effectively performing the task at hand, I'd expect a butcher's bill of at least a million. Minimum.