Cheapest priced copy (Tenth Edition): $0.35 at Card Kingdom, near mint, their price list as of .
Llanowar Sentinel 
Creature — Elf · 2/3 · Common · Tenth Edition · #275
When this creature enters, you may pay {1}{G}. If you do, search your library for a card named Llanowar Sentinel, put that card onto the battlefield, then shuffle.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Tenth Edition | 275 | Common | $0.35 | $0.49 |
| Weatherlight | 134 | Common | $0.35 | — |
2 rulings from Wizards of the Coast
- 4 October 2004Because the "search" requires you to find a card with certain characteristics, you don't have to find the card if you don't want to.
- 15 July 2007Llanowar Sentinels can "chain": If you put a second Llanowar Sentinel onto the battlefield as a result of this ability, that creature's "enters" ability triggers and allows you to search for yet another Llanowar Sentinel.