#You're going to need to reference your current map document and the data frame, assuming you just have one
mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd)[0]

#Associate scale bars in your layout with variables that arcpy can manipulate
m_scale = arcpy.mapping.ListLayoutElements(mxd, "MAPSURROUND_ELEMENT", "m scale bar")[0]
km_scale = arcpy.mapping.ListLayoutElements(mxd, "MAPSURROUND_ELEMENT", "km scale bar")[0]

#Iterate through all the pages, for each page, look at the scale, then adjust the scale bars to be on, or off, the page.
for page in range(1, mxd.dataDrivenPages.pageCount +1):

    #Check the scale and move the elements around to get the right scale bar on the page
    if df.scale < 25000:
        m_scale.elementPositionX = 5 #on the page
        km_scale.elementPostitionX = 15 #off the page

        #Export the current page to a pdf, using a specified path
        arcpy.mapping.ExportToPDF(mxd, r"C:ProjectOutputProject1.pdf", df)
    else:
        m_scale.elementPositionX = 15 #off the page
        km_scale.elementPostitionX = 5 #on the page

        #Export the current page to a pdf, using a specified path
        arcpy.mapping.ExportToPDF(mxd, r"C:ProjectOutputProject1.pdf", df)
    mxd.dataDrivenPages.refresh()