Cheapest priced copy (Mystery Booster 2): $9.49 at Card Kingdom, near mint, their price list as of .
Umbral Mantle 
Artifact — Equipment · Uncommon · Shadowmoor · #267
Equipped creature has "{3}, {Q}: This creature gets +2/+2 until end of turn." ({Q} is the untap symbol.)
Equip {0}
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $9.49 to $119.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mystery Booster 2 | 101 | Uncommon | $9.49 | — |
| The List | SHM-267 | Uncommon | $11.99 | — |
| Shadowmoor | 267 | Uncommon | $14.99 | $119.99 |
3 rulings from Wizards of the Coast
- 1 May 2008When you activate an {Q} ability, you untap the creature with that ability as a cost. The untap can't be responded to. (The actual ability can be responded to, of course.)
- 1 May 2008If a creature with an {Q} ability hasn't been under your control since your most recent turn began, you can't activate that ability, unless the creature has haste.
- 1 May 2008If the permanent is already untapped, you can't activate its {Q} ability. That's because you can't pay the "Untap this permanent" cost.