Iberia is selling San Francisco to Munich tickets for Oneworld flights on American and British Airways in September and October as low as $410 round trip. Use Skyscanner or Momondo and there are ticket prices as low as $380 round trip with online travel agencies like Hop2.com. These are carry-on only fares.
Munich Oktoberfest runs Sat Sep 21 through Sun Oct 6.
Iberia ticket $410.93 (Google Flights does not have Iberia deep link. Find low fare dates and go to Iberia to book).
San Francisco SFO – London LHR – Munich MUC – London LHR – San Francisco SFO
Sunday Sep 8 – Sun Sep 22
Even lower prices can be found through online travel agency sites. Sep 8-22 prices at $379.80 on Hop2.com, but means a long 9 hour layover in Charlotte. Other itineraries like the one below have better schedules.
San Francisco SFO – London LHR – Munich MUC – Dallas DFW – San Francisco SFO
Tue Oct 1 – Tue Oct 15