Cheapest priced copy (Return to Ravnica): $0.35 at Card Kingdom, near mint, their price list as of .
Security Blockade 
Enchantment — Aura · Uncommon · Return to Ravnica · #20
Enchant land
When this Aura enters, create a 2/2 white Knight creature token with vigilance.
Enchanted land has "{T}: Prevent the next 1 damage that would be dealt to you this turn."
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Return to Ravnica | 20 | Uncommon | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 1 October 2012You must target a land to cast Security Blockade. If that land is an illegal target when Security Blockade tries to resolve, it won't resolve and won't enter. You won't get the Knight token.