Firstly, you might consider flights from LAX to MRS via LHR, AMS, or FRA. The Marseille airport is no farther from Aix-en-Provence than it is from Marseille, about 28 km. Often, tickets via a stopover somewhere can cost much less than nonstops. If your destination is indeed Aix-en-Provence, a ticket via some European airport to MRS will probably cost less than a BCN nonstop and a subsequent, separate train connection.
Secondly, as it remains very difficult for US passport holders to enter France, service options are very limited resulting in increased fares. Once France opens its borders, evidence presently points to early 2022, more flights will be reinstated and prices should ease.