Boston to Southwest Illinois Flights (BOS-STL) from $125

Booking well in advance is one of the easiest ways to cut the cost of your journey from Boston to Southwest Illinois. In most cases, the further ahead you book, the cheaper flights tend to be. Consider traveling in the off-season, too, when prices are often lower across the board. For flights to Southwest Illinois, the cheapest month is usually October.* Great value flights can be had on this route departing from Boston—and Travelocity's wide range of options is sure to include one that works for your family.

Generally speaking, flights from Boston to Southwest Illinois are usually cheapest in October.* In other words, that's the month to travel if you're looking to save on your adventure. Don't forget that flights are often cheaper if you book them in advance, too: Planning your travel well ahead of time is a great way to keep the cost of your family vacation low.

Bookings for flights from Boston to Southwest Illinois generally open 12 months in advance. If you're an early bird and book as soon as your date opens, that means you have a full year to sort out all the reservations for your hotel, hire car, and family-friendly vacation activities. Sometimes time's a bit tighter—but rest assured that you'll often still be able to find last-minute flights from Boston to Southwest Illinois right up to the day that your plane is due to take off.

Whether you book one-way or return flights is a question of personal preference. Often, booking return flights can be a cheaper option. For instance, round-trip flights from Boston to Southwest Illinois in 2026 are available from as little as from $260, while one-way journeys start at from $125.* However, one-way flights give you the freedom to travel back from a different city or book your return leg at a later point in time, so you can shape your trip to suit you and your family.

When you book with Travelocity, you can choose from a large selection of airlines for your trip from Boston to Southwest Illinois. With such an array to choose from, you're sure to find one that works for the whole family. Plus, many Travelocity flight bookings come with free cancellation—and should you need a helping hand, the customer service team is available 24/7 to assist you.