Cheapest priced copy (Dragons of Tarkir): $0.35 at Card Kingdom, near mint, their price list as of .
Scaleguard Sentinels 
Creature — Human Soldier · 2/3 · Uncommon · Dragons of Tarkir · #201
As an additional cost to cast this spell, you may reveal a Dragon card from your hand.
This creature enters with a +1/+1 counter on it if you revealed a Dragon card or controlled a Dragon as you cast this spell.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dragons of Tarkir | 201 | Uncommon | $0.35 | $0.49 |
| Dragons of Tarkir Promos | 201 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 25 February 2015If you don't reveal a Dragon card from your hand, you must control a Dragon as you are finished casting the spell to get the bonus. For example, if you lose control of your only Dragon while casting the spell (because, for example, you sacrificed it to activate a mana ability), you won't get the bonus.
- 25 February 2015If one of these spells is copied, the controller of the copy will get the "Dragon bonus" only if a Dragon card was revealed as an additional cost. The copy wasn't cast, so whether you controlled a Dragon won't matter.
- 25 February 2015You can't reveal more than one Dragon card to multiply the bonus. There is also no additional benefit for both revealing a Dragon card as an additional cost and controlling a Dragon as you cast the spell.