Cheapest priced copy (Duel Decks: Knights vs. Dragons): $3.99 at Card Kingdom, near mint, their price list as of .
Knight Exemplar 
Creature — Human Knight · 2/2 · Rare · Magic 2011 · #20
First strike (This creature deals combat damage before creatures without first strike.)
Other Knight creatures you control get +1/+1 and have indestructible. (Damage and effects that say "destroy" don't destroy them.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
8 printings · $3.99 to $9.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duel Decks: Knights vs. Dragons | 14 | Rare | $3.99 | — |
| Magic 2011 | 20 | Rare | $4.99 | $9.99 |
| March of the Machine Commander | 192 | Rare | $3.99 | — |
| The List | DDG-14 | Rare | $4.49 | — |
| The List | M11-20 | Rare | $3.99 | — |
| Magic 2011 Promos | A7 | Rare | — | $5.49 |
| Secret Lair Drop | 707 | Rare | $6.99 | $4.99 |
| Secret Lair Drop | 1044 | Rare | $7.99 | $7.49 |
4 rulings from Wizards of the Coast
- 15 August 2010If an effect would simultaneously destroy Knight Exemplar and another Knight creature you control, only Knight Exemplar is destroyed.
- 1 July 2013Lethal damage, damage from a source with deathtouch, and effects that say “destroy” won't cause a creature with indestructible to be put into the graveyard. However, a creature with indestructible can be put into the graveyard for a number of reasons. The most likely reasons are if it's sacrificed, if it's legendary and another legendary creature with the same name is controlled by the same player, or if its toughness is 0 or less.
- 1 July 2013If you control two Knight Exemplars, each one causes the other to get +1/+1 and have indestructible.
- 1 July 2013If another Knight creature you control is dealt lethal damage, the creature isn't destroyed, but the damage remains marked on it. If, at some point later in that turn, you no longer control Knight Exemplar or it loses its abilities, the other Knight creature will lose indestructible and will be destroyed.