Bealine
I have little doubt that it is figure plucked at random.
Your student might be able to get a copy of Beyond Airline Disruptions by Jasenka Rapajic (ISBN 978-0-7546-7440-5) from his/her university library. There is an interesting table on P16 giving various estimates. One of the highest is for Euro 600 per wide bodied jet per minute from a 'Major European Airline'.
There are several types of cost:
Direct (staff overtime, cost of running APU, etc)
Reactionary - having to re-book passengers who miss connections, etc
Loss of revenue if the passenger decides to use another airline next time
The cost to the passenger of lost time (as used by economists for cost benefit analysis but not actually a cost to the airline).
Many of these will only kick in at a certain point.
There has been considerable schedule creep in recent years. Compare block times between LHR & CDG or AMS now and 20 years ago. Of course they are not delays (actually padding to reduce delays), but as an exercise apply the £300 per minute to the length that the schedule has been extended).