Cheapest priced copy (World Championship Decks 1999): $0.69 at Card Kingdom, near mint, their price list as of .
Temporal Aperture 
Artifact · Rare · Urza's Saga · #310
{5}, {T}: Shuffle your library, then reveal the top card. Until end of turn, for as long as that card remains on top of your library, play with the top card of your library revealed and you may play that card without paying its mana cost. (If it has X in its mana cost, X is 0.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.69 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Saga | 310 | Rare | $5.99 | — |
| World Championship Decks 1999 | kb310 | Rare | $0.69 | — |
3 rulings from Wizards of the Coast
- 4 October 2004The card is removed from the top of the library when you play it, just like a card is removed from your hand when you play it.
- 4 October 2004While you do not have to pay the mana cost, you do have to pay any other costs described in the text that are paid when playing the card. If the cost in the card text is optional, such as with Buyback, you can optionally pay that cost.
- 4 October 2004If your library is shuffled or that card otherwise leaves the top of your library, the effect ends.