Call a human travel agent, pay their fee and let them scour for fares. You don't have to fly on PR to get to SFO when you can fly CX via HKG or many other options. D fares on PR non stop are US$2,782 + tax/fee if available. For a flight that long, non-stop, one-way that sounds pretty typical. Round trip in I is US$3,120 so you can see one-way is going to cost.
Are you looking for round trip, and non-stop? What's "expensive"?