Cheapest priced copy (Morningtide): $0.49 at Card Kingdom, near mint, their price list as of .
Mothdust Changeling 
Creature — Shapeshifter · 1/1 · Common · Modern Masters · #53
Changeling (This card is every creature type.)
Tap an untapped creature you control: This creature gains flying until end of turn.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.49 to $8.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Masters | 53 | Common | $0.59 | $4.99 |
| Morningtide | 42 | Common | $0.49 | $8.99 |
| The List | MOR-42 | Common | $0.59 | — |
One ruling from Wizards of the Coast
- 1 April 2008Since the activated ability doesn't have a tap symbol in its cost, you can tap a creature (including Mothdust Changeling itself) that hasn't been under your control since your most recent turn began to pay the cost.