OK, that wasn't clear to me from the post. But yes, I agree, that difference seems a bit absurd. Typically there is a premium to booking an AA codeshare vs. other airline prime flight, in part due to the incremental benefit you get, mainly in terms of mileage accrual (e.g., most CX economy fares don't earn anything on AA). While I'm not sure if this itinerary is booked as an AA codeshare, here you obviously get the benefit of not having to re-check your baggage and potentially more seamless re-accommodation in case of a misconnect. If it's booked as an HA segment, the reason it would be so expensive is that there probably isn't a discounted fare that allows for the combination of AA- and HA-marketed segments, so it's bumping you up to a full fare or near-full fare.