Cheapest priced copy (Avacyn Restored): $0.49 at Card Kingdom, near mint, their price list as of .
Lunar Mystic 
Creature — Human Wizard · 2/2 · Rare · Avacyn Restored · #65
Whenever you cast an instant spell, you may pay {1}. If you do, draw a card.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Avacyn Restored | 65 | Rare | $0.49 | $0.49 |
| Commander 2020 | 115 | Rare | $0.49 | — |
2 rulings from Wizards of the Coast
- 1 May 2012The triggered ability will go on the stack on top of the instant spell and resolve before it.
- 1 May 2012The decision to pay {1} is made when the triggered ability resolves. It’s not an additional cost of the instant spell.