As others have pointed out, flights across the Pacific are the best way to earn PQM at the lowest cost. The reason being is that for now, there is no Basic Economy across the Pacific. Hence you'll earn 100% PQM and 1 PQS per segment flown, even on the cheapest fare. I'd advise against doing domestic trips since you'll run into Basic Economy fares which only earn 50% of the miles and only give you 0.5 PQS not 1 PQS as you may expect. You may want to look into flying into the Eastern part of Canada on UA (which will earn 100% PQM and 1 PQS per segment since transborder flights don't have Basic Economy yet), then start from there to Asia on UA. Key airports to look out for would be YYZ, YOW and YUL. IIRC a roundtrip from California to those cities is about 4000 miles. Add on your TPAC and you've got another 21000 miles there for 25000. Which leaves you with another 5K PQM to earn. Perhaps another a roundtrip to North East Canada (this time with a silly routing to boost PQM?)
Safe Travels,
James