As for the breakdown of the rewards, first off there is the proceeds of the sale of the killed flying dragon.
Since the flying dragon is a dragon after all, parts of its body are well in range of being worth more than a platinum coin.
There are also the proceeds of sale for the specially made armor the flying dragon had equipped.
Of course, since it couldn’t be used as is, it has to be melted down. But that armor’s exterior is coated in mithril.
Even that much guarantees a decent value, I was told.