Flying into one city and home from another can save backtracking. It can also hide a journey that the flight booking does not arrange for you. The space between those airports belongs in the budget before the fare is called a bargain.
Draw the missing connection
Write the arrival city, departure city and the dates between them. Then identify how you intend to travel across the gap. A train, coach, rental car or separate flight each has its own booking, luggage and timing constraints.
Do not assume that a multi-city search result includes ground transport or protection for a separately arranged connection. Read the actual ticket and booking terms, including which parts belong to one reservation.
Compare complete trips
Make two simple totals: the return-flight plan with its backtracking costs, and the open-jaw plan with its onward travel costs. Include station or airport transfers, luggage charges where relevant and any extra overnight stay.
Time matters too. An apparently cheaper departure airport may require leaving a destination the evening before, turning the last day into a transfer day. Price the itinerary you are willing and able to follow, not the shortest connection a search engine can display.
For an illustrative comparison, a lower airfare is not a saving if the added transport and accommodation exceed the difference. Use current quotes for your actual dates; there is no fixed amount that makes every open-jaw booking worthwhile.
Check entry and transit requirements for the real route through official sources before committing. A different airport or overland crossing can change the journey’s requirements.
Finally, leave a sensible margin before the homeward flight and understand what happens if the independently booked transfer is delayed. The strongest itinerary is the one whose missing connection has been planned, priced and checked rather than left as a line between two attractive cities.