Cheapest priced copy (Theros): $0.35 at Card Kingdom, near mint, their price list as of .
Nylea's Disciple 
Creature — Centaur Archer · 3/3 · Common · Theros · #167
When this creature enters, you gain life equal to your devotion to green. (Each {G} in the mana costs of permanents you control counts toward your devotion to green.)
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $3.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros | 167 | Common | $0.35 | $3.49 |
4 rulings from Wizards of the Coast
- 15 September 2013Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).
- 15 September 2013Numeric mana symbols ({0}, {1}, and so on) in mana costs of permanents you control don't count toward your devotion to any color.
- 15 September 2013Mana symbols in the text boxes of permanents you control don't count toward your devotion to any color.
- 15 September 2013If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it's still on the battlefield at that time.