Say you find $100, You don't know whether that's D or 2D, so, as far as you're concerned, it's 1.5D. If you also know that there are no $100/200 envelopes, then knowing the $100 matters. But the statement of the game does not reveal that fact, so if we go with your "reasonable" assumption that any amount you discover has a 50% chance of being D and a 50% chance of being 2D, those probabilities don't change just because you know the amount.

Knowing the amount in the envelope only matters if you know the distribution of envelopes. Otherwise, knowing the amount adds no information.