Stu,
By going into the negotiation with one or other saying their opposite has to justify everything, this could take forever and comes across as belligerence.

Of course they have to justify the sum they are demanding.
Of course we should only pay what we consider we rightfully owe.

We are leaving them, not the other way around so the onus is on us to make sure we reach a compromise,

When you check out of an hotel, if you consider the bill excessive, you go through the itemised bill and check that you agree with what you are being charged for.

An itemised bill is expected.