Here is something I threw together really quick. It ran fine for me and properly inverted the values of the input raster. import arcpy
from arcpy.sa import *
arcpy.CheckOutExtension("Spatial")
InputRaster = "\\c:\\temp\\Input.img"
OutputRaster = "\\c:\\temp\\Output.img"
RasterMax = arcpy.GetRasterProperties_management(InputRaster,"MAXIMUM")
OutputRasterMax = (RasterMax.getOutput(0))
RasterMin = arcpy.GetRasterProperties_management(InputRaster, "MINIMUM")
OutputRasterMin = (RasterMin.getOutput(0))
RasterInvert = ((Minus(InputRaster, float(OutputRasterMax))*(-1)+ float(OutputRasterMin)))
RasterInvert.save(OutputRaster)