Excess Baggage Charges

What is the average charge per kilo for excess baggage?

My family are currently on their way to Heathrow with Qatar Airways on flight QR001. They came from Bangkok and had a quick stop-over in Doha, Qatar.

This isn’t the first time they’ve used Qatar and certainly not the first time they’ve been very slightly overweight on their baggage, but the airline has just charged them £96 to carry the extra.

If you were paying £96, how much extra baggage would you expect. Unfortunately, at the moment, I don’t know how much extra baggage they had. They left with practically full suitcases so not much else could have been added to them!