Is that an absolute dollar amount regardless of how many miles you might get?
I would think the dollar amount generally changes based on how many miles are in question.
Yes and no. If one has multiple alliances then the discretionary number can decrease to meet one of those options. For the sake of the miles associated with a YYZ-NRT r/t ticket I'd probably spend the extra $175 for my carrier of choice, especially knowing that I had no vested interest in the alternative. In my personal situation, where either ST or *A are valid earning avenues, however, I wouldn't spend a penny extra, as the *A miles are just as valuable (if not more) than the ST miles to me personally.