Havana:
You're quite right, but...
I did a little experiment.
I'd assume that a Friday flight would be (significantly) busier than a Tuesday flight.
I'd assume that by now (Thursday), virtually all the passengers who are going to fly tomorrow have booked, so the fare on offer for tomorrow morning's flight is about as high as it's going to get.
That said, eujet.com is quoting me £54.50 plus taxes for going out MSE-AMS tomorrow. I fear that 30 x £54.50 would barely cover the operating costs, let alone the overhead. And in practice, £54.50 is the highest (or nearly the highest) fare - the average will be a lot lower. I note that AMS-MSE next Wednesday (for example) is £7.00 plus tax.
The tradeoff with no-frills carriers is that the fare may be lower but the breakeven seat factor is higher. It's definitely not below 30%! Twice that (60%) would be more like it, I reckon (I'm being optimistic, allowing for the low ownership cost of the aircraft and the fact that typically, of the "taxes and charges" collected in addition to the basic fare, not all actually gets paid out in taxes and charges due to discounts).
C.