Cheapest priced copy (Duel Decks: Elves vs. Inventors): $0.35 at Card Kingdom, near mint, their price list as of .
Elvish Branchbender 
Creature — Elf Druid · 2/2 · Common · Lorwyn · #204
{T}: Until end of turn, target Forest becomes an X/X Treefolk creature in addition to its other types, where X is the number of Elves you control.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Elves vs. Inventors | 6 | Common | $0.35 | — |
| Lorwyn | 204 | Common | $0.35 | $0.79 |
3 rulings from Wizards of the Coast
- 1 October 2007The value of X is determined when the ability resolves. It won’t change later, even if the number of Elves you control changes.
- 1 October 2007If you control no Elves when the ability resolves, the Forest will become a 0/0 creature and be put into the graveyard. However, since Elvish Branchbender is itself an Elf, the Forest will usually be at least a 1/1.
- 1 August 2008A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.