The new Miracle mechanic causes a replacement effect for the casting cost of the card, which must be revealed and played immediately. How does this interact with flashback? If a player plays a card for its Miracle cost, what is the flashback cost of the card if it's flashed back with something like Snapcaster Mage?
Snapcaster Mage: Bold for emphasis.
The flashback cost will be the mana cost of the card, shown on the top right corner of the card. Alternative costs such as the Miracle cost are irrelevant.
But what if you had a Bösium Strip in play?
The Miracle cost may only be payed if that card is the first card drawn from your library in a turn, and only when you draw the card. Clearly if the card is in your graveyard, it cannot be drawn, so the replacement effect for the casting cost will never apply.
As an aside, note the implications for a card like Future Sight. If you play the top card in your library that's revealed, you won't be drawing it, so you must pay full cost.
Miracle is an ability that only applies as you draw the card. This has no interaction with casting from the graveyard. (I can't speak for future MTR, but I'm assuming judges won't let you pay the miracle cost if you've already let the card touch your other hand cards, period.)
Furthermore, the general rule is that alternative costs don't mix.
It's only by paying the alternative cost for flashback that you are allowed to play the spell from your graveyard in the first place; normal cost modifiers (like Thorn of Amethyst) and additional costs (like kicker) still apply, but the base cost is the flashback cost and only the flashback cost, not the miracle cost or madness cost or any other alternative cost. A Mindbreak Trap you've given flashback with Snapcaster Mage, for example, will still cost 2UU to play even in circumstances where it would cost 0 to play from your hand (due to its trap ability).