How to get sum of table column values into Python variable?

summed_total = 0
with arcpy.da.SearchCursor(fc, "field to be totaled") as cursor:
    for row in cursor:
        summed_total = summed_total + row[0]

Something like this would work. Replace what's in quotes with your field name, or with a list of fields you're going to be working with. Replace fc with the feature that holds the field.

That won't work for 10.0 or earlier; da.SearchCursor didn't show up until 10.1. For earlier versions:

summed_total = 0
field = "field to be summed"
with arcpy.SearchCursor(fc):
    for row in cursor:
        summed_total = summed_total + row.GetValue(field)

You can accomplish this in two lines when working with numpy arrays (TableToNumPyArray).

import arcpy, numpy

fc = r'C:\path\to\your\fc'

# Convert to numpy array.  "test" is the field name
field = arcpy.da.TableToNumPyArray (fc, "test", skip_nulls=True)
sum = field["test"].sum()