I understand that the Foreign Office has ALWAYS charged the "equivalent commercial fare" for any evacuation they arrange from troublespots, so no difference to previous procedures over the years. If you haven't got the cash, as so often in such circumstances, they will still carry you, but take your passport (which is their's, not yours) when you get back, until you pay up. I presume all these rules etc have long been in place.
As it is traditionally the Standard airline fare which is charged (because this concept comes from long before the days of Yield Management), and having a look at BA's standard fares over the next week or so to a comparable destination, Tel Aviv (Cairo not providing appropriate fares at the moment, for obvious reasons), the one-way fare is £659. So charging £300 seems something of a bargain. I presume £300 x 100 pax (because they are apparently nothing like full) = £30,000 comes nowhere near the 757 round trip charter costs.