What it looks like
Three days in Lisbon, added to the end
from the example plan- Lisbon from Madrid, where the trip already ends
- $35 per person
- The same city from Dallas
- $591 per person, about 17× more
- Out + home from Lisbon, minus the return you already had
- +$217 per person
- What that buys
- Three extra days and a different country
The $35 fare is the easy half. The number that matters is the $217, because the way home is what most flight sites never price: they show you the cheap hop out and stop there.
When it does not work
- The candidate list is hand-picked. Somewhere genuinely cheap that is not on it will not be found.
- The marginal cost assumes three extra days and a flight home on that date. Change either and the number changes.
- Rows that were not verified live are cached-fare estimates and say so on the card. Check them before you count on them.
- Cheap flights are not a cheap trip: three days somewhere is still three days of hotels, food, and a visa if one is needed.
- Anything already on your route, or in your own country, is excluded; that is not a discovery.
Risk
Low risk
These are ordinary tickets. The only exposure is committing to a separate return before the outbound is confirmed.