A couple of days ago, Southwest Airlines opened up their schedule for
December bookings, and the fares are pretty good. I'm in L.A., and the
fares from here to Nationals in St. Louis were about $260 r/t.

The last time I checked, the airfares to SLC and Seattle (drive to
Vancouver) for the World Cups were very reasonable, too...

If you're in a city serviced by Southwest, I'd also recommend signing
up for their 'DING' alerts... if a great deal shows up at the last
minute, you can grab it - or if you've already bought a ticket, you can
re-book and they'll give you a credit for the difference. You can use
the credit within one year toward a future SW flight.