Just shows you it always pays to shop around.

Ticket stock restrictions are definitely possible, but haven't seen mentioned the potential for a codeshare to be priced differently than the operating carrier does. This has always been the case - as a codeshare (UA flight XXXX instead of Copa YYY) - UA markets the flight and decides what they are willing to sell it for, independent of what Copa thinks. For whatever reason, its possible that UA believes it can get double the price then Copa thinks (and if they are right, Copa is leaving a lot of $ on the table).