How the planner finds it
- A curated list of about forty destinations — Tokyo, Bangkok, Istanbul, Reykjavík, Marrakech, Mexico City, São Paulo and the like — is priced from your last city and from home, out of cached fares for the right month.
- The “surprise” number is how many times cheaper it is from your route than from home. Under 1.4× is dropped, and anything actually cheaper from home is dropped outright.
- The number that matters is the marginal cost: fare out to the add-on, plus fare home from the add-on, minus the return flight you already had. That is what the extra days really cost.
- A negative marginal cost means the longer trip is cheaper than the shorter one. It happens, and when it does the card shows a minus sign, not a plus.
- Anything over $700 marginal is dropped as an add-on nobody takes, though it can still appear as a “cheap from here” surprise.
- The best few are verified with two live searches each — out and home — and marked verified. The rest stay labelled as cached-fare estimates.
- Add-ons are measured from the last city only, because that is the only place the marginal cost is honest. Other cities on the route can show a surprise price with no marginal number attached.
Worked example
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
- +$180 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 $180, 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.