KLM Physically Challenged Passengers
The link, easily found, on the on the KLM website clearly explains what is needed in order to have oxygen supplied during flight. It states that there is a standard charge for this, but doesn't say how much. One is to phone their special number to find out that detail.
I am not at all surprised that KLM, or any airline for that matter, would charge for supplying oxygen. Maybe if the couple had done some simple research they wouldn't have been so keen to unleash their fury via the Daily Mail.