Cheapest priced copy (Darksteel): $3.99 at Card Kingdom, near mint, their price list as of .
Spellbinder 
Artifact — Equipment · Rare · Darksteel · #143
Imprint — When this Equipment enters, you may exile an instant card from your hand.
Whenever equipped creature deals combat damage to a player, you may copy the exiled card. If you do, you may cast the copy without paying its mana cost.
Equip {4}
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $3.99 to $14.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Darksteel | 143 | Rare | $3.99 | $14.99 |
5 rulings from Wizards of the Coast
- 1 December 2004Spellbinder's second ability allows you to create a copy of the imprinted instant card in the Exile zone (that's where the imprinted instant card is) and then cast it without paying its mana cost.
- 1 December 2004Spellbinder imprints a card only once, when it enters. Attaching Spellbinder to a creature doesn't trigger its "enters" ability.
- 1 December 2004You don't pay the spell's mana cost. If the spell has X in its mana cost, X is 0. You do pay any additional costs for that spell. You can't use any alternative costs.
- 1 December 2004If the copied card is a split card, you choose which one side of it to cast, but you can't cast both sides. You can choose the other side later in the game, though.
- 1 March 2005The creation of the copy and then the casting of the copy are both optional.