The Zimbabwe price is quoted in $us so I don't think the devaluation of the Zim $ will have made any difference. I think that the ticket has to be paid for with foreign currency as well. There is a black market in Zimbabwe but I don't think you can go in with a wad of US dollars, change it on the black market and then waltz into the BA office and buy a Oneworld ticket without exchange receipts confirming your currency transaction at the official rate.