Matter Reshaper is only just launching this week, though I think it's a great card to talk about in terms of price trajectory. This is a card that has seen a good amount of buzz, and is preordering for a whopping $8 from SCG. I highlighted it as an overhyped card in my most recent Insider article, and we're starting to see that come to fruition. TCGPlayer has launched sales for Oath of the Gatewatch cards, and the price has already fallen sub-$6.
Players seem to be realizing that actually supporting Matter Reshaper in the Rally the Ancestors deck isn't as simple as they predicted, and the buzz is wearing off. With the coming Pro Tour being Modern, there's a lot of pressure on Matter Reshaper to see a lot of success on the SCG Tour in the opening weeks, and it will likely tank if it fails to deliver.
As an Eldrazi, it does have potential to appear in Modern Eldrazi decks, but that doesn't generate near the demand that Standard success does for newly printed cards. If you're the type to purchase sealed product and/or if you've cracked some of these in prize packs, I strongly recommend putting them right into your trade binder and looking for people who want them. Even something like a $2-3 buylist would make me happy for this card. Unless this card Top 8s SCG Atlanta this weekend, you'll be sorry that you held onto yours, and even if it does it still has room to fall as a regular rare.