I tried a sample SFO-LIM itinerary for Nov, and a couple interesting restrictions showed up in the fare rules:
Code:
DOUBLE OPEN JAWS NOT PERMITTED.
END-ON-END
END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE
COMBINATIONS.
PROVIDED -
COMBINATIONS ARE FOR CARRIER UA/LH/LX/OS/AC/SN IN
ANY RULE
FOR TRAVEL VIA THE ATLANTIC.
COMBINATIONS ARE FOR CARRIER UA/NH IN ANY RULE
FOR TRAVEL VIA THE PACIFIC.
OPEN JAWS
FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
-TO FORM SINGLE OPEN JAWS
MILEAGE OF THE INTERNATIONAL OPEN SEGMENT MUST BE
EQUAL/LESS THAN MILEAGE OF THE LONGEST FLOWN FARE
COMPONENT. THERE IS NO MILEAGE RESTRICTION ON AN
OPEN SEGMENT WITHIN ONE COUNTRY.
PROVIDED -
WHEN THE OPEN SEGMENT OCCURS
-WITHIN AREA 1 COMBINATIONS ARE WITH ANY FARE FOR
CARRIER UA IN ANY RULE AND TARIFF.
ROUND TRIPS/CIRCLE TRIPS
FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
-TO FORM ROUND TRIPS/CIRCLE TRIPS.
PROVIDED -
COMBINATIONS ARE WITH ANY FARE FOR CARRIER UA IN
ANY RULE AND TARIFF.
OTHERWISE
END-ON-END COMBINATIONS PERMITTED. VALIDATE ALL FARE
COMPONENTS. SIDE TRIPS PERMITTED WITH NO
RESTRICTIONS. TRAVEL MUST BE VIA THE POINT OF
COMBINATION.
So I don't think this fare allows SFO-LIM/SFO-CDG-SFO itinerary, as SFO-LIM/SFO-CDG is double-open-jaw. If you try, I'm thinking it turns your outbound half-roundtrip fare into SFO-LIM one-way, which may be a few dollars more than the complete roundtrip. Seems to be consistent with what you are reporting.
I think what you need is some method where the outbound doesn't reprice. So I'm wondering if you can do SFO-LIM/XXX-SFO (where the open jaw is shorter than SFO-LIM 4500mi, ie LAX)? You might need to buy a one-way outbound on a separate PNR to make this work. If it prices lower, I think you can get the difference back as TCVA, which you can use at your leisure (typ 1yr from TCVA issuance).
Fare rules are infinitely more complex than we think, and combining them makes things even worse. So I'm not sure if any of this will work. Then again, the cost of the short MR plus change fee may be $400, so out of the $650ish potential, your net would be $250, which is hardly worth it. But if you need to go out to LAX (or any of 90%+ of US destinations) anyway, this may be more reasonable to do, as your cost would only be the change fee.
Good luck!