For your information, there are refundable fares (usually designated as FLEXI-something) even on low-cost carriers, so in that sense even a fully booked flight is not necessarily 100% paid for, unless everybody really shows up
I have not researched this but I am pretty sure that no LOCO fares are refundable, what they often are is transferable to another flight. The tax element is refundable, subject to an administration fee which often exceeds the value of the refund!