This is actually a very complicated subject and one which people do not think about. When a flight is cancelled the ground staff are working under very stressful conditions , so most airlines have pre agreed arrangements to accept each others tickets, and the revenue is eventually (can take months) transferred through the IATA clearing house.
The actual amount depends on whether the ticket is endorsed at face value (usually no change in routing or additional sectors) which means the receiving airline get the value of the coupon ,the ticket can also be involuntary re-routed (similar to above however some arilines have pre-set agreements on sector value). A flight interuption manifest can also be issued which allows the receiving airline to initially claim the full fare this can then be adjusted to reflect the actual fare paid but can again take months to sort out. Basically it is an expensive business when a flight is cancelled.