Offline SMPS EMC scan: Why is "common mode only" scan so expensive?

[ocset]

Hi,
We find, when we go to get conducted EMC scans (of <200W offline SMPS’s) done at EMC test places in UK, they want big money to do individual common mode or differential mode conducted scans.
So we end up with only having EMC scans which are a mixture of the common mode and diff mode disturbances. (my company can't pay for anything better).

We realise that to get a conducted , "common mode only" scan, we need to have the EMC scan done with a splitter in place, and the EMC receiver has to then do Magnitude spectral measurements.

However, we also believe that an even easier way to get “common mode only” conducted scans, is to use the EMI receiver to measure live and neutral simultaneously…and then post-process tha scan data. I am certain that our EMC test house does this anyway, because their EMI receiver (network analyser) always does just one scan, but they give us separate live and neutral scans from the one scan…therefore, they must be scanning live and neutral simultaneously………their EMI receiver goes straight into a computer, so I don’t see why they want to charge us so much more for “common mode only” scans?
Do you know why “common mode only” scans are so much more expensive?