Cheapest priced copy (Commander 2016): $0.49 at Card Kingdom, near mint, their price list as of .
Spelltwine 
Sorcery · Rare · Magic 2013 · #68
Exile target instant or sorcery card from your graveyard and target instant or sorcery card from an opponent's graveyard. Copy those cards. Cast the copies if able without paying their mana costs. Exile Spelltwine.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $0.49 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2016 | 97 | Rare | $0.49 | — |
| Commander 2017 | 94 | Rare | $0.59 | — |
| Duel Decks: Jace vs. Vraska | 34 | Rare | $0.49 | — |
| Magic 2013 | 68 | Rare | $0.49 | $2.49 |
7 rulings from Wizards of the Coast
- 1 July 2012The copy you cast first will already be on the stack when you cast the other copy. If that spell targets a spell, you may choose the first copy as a legal target.
- 1 July 2012If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.
- 1 July 2012If one of Spelltwine’s targets is illegal when Spelltwine tries to resolve, you’ll still exile and copy the remaining legal target.
- 1 July 2012Spelltwine has two targets: the instant or sorcery card in your graveyard and the one in an opponent’s graveyard. You can’t cast Spelltwine unless you can choose both legal targets.
- 1 July 2012You can cast the copies in either order.
- 1 July 2012If the card has X in its mana cost, you must choose 0 as its value.
- 1 July 2012The copies are created in exile and cast from exile. The cards remain exiled.