Lufthansa still has flights under $400 round trip for travel from SFO to Lisbon, Portugal in May. You can even return in mid-June for around $400. This fare is a great deal if you can travel with only a carry-on bag and you do not care about earning frequent flyer miles.
Lufthansa SFO – Munich – Lisbon – Munich – SFO
I am heading to Lisbon this summer on a $470 round trip ticket with Air Canada I purchased in January. Ticket prices for my dates are now in the $1,300 range.