Cheapest priced copy (Commander 2013): $0.99 at Card Kingdom, near mint, their price list as of .
War Cadence 
Enchantment · Uncommon · Mercadian Masques · #224
{X}{R}: This turn, creatures can't block unless their controller pays {X} for each blocking creature they control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.99 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2013 | 128 | Uncommon | $0.99 | — |
| Mercadian Masques | 224 | Uncommon | $0.99 | $11.99 |
2 rulings from Wizards of the Coast
- 4 October 2004The ability only applies to blocks declared after it resolves. It will not add costs to any blockers already announced.
- 4 October 2004You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t block unless the controller chooses to pay the cost (a cost of zero is not automatically paid). You can use this to make your creatures unable to block an attacker.