Cheapest priced copy (Commander 2018): $0.49 at Card Kingdom, near mint, their price list as of .
Prototype Portal 
Artifact · Rare · Commander 2018 · #216
Imprint — When this artifact enters, you may exile an artifact card from your hand.
{X}, {T}: Create a token that's a copy of the exiled card. X is the mana value of that card.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.49 to $5.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2018 | 216 | Rare | $0.49 | — |
| The List | SOM-195 | Rare | $0.99 | — |
| Scars of Mirrodin | 195 | Rare | $0.49 | $5.99 |
5 rulings from Wizards of the Coast
- 1 January 2011If the exiled card has {X} in its mana cost (such as Chalice of the Void), that X is considered to be 0. The {X} in Prototype Portal's activation cost takes this into account, though it may be greater than 0 if the exiled card has other mana symbols in its mana cost (such as Riptide Replicator).
- 1 January 2011You may not activate the second ability if no card has been exiled with Prototype Portal. In that case, the value of {X} is undefined and can't be paid.
- 1 January 2011You don't choose the value of {X}. Rather, the value of X is defined by the activated ability.
- 1 January 2011Any "enters" abilities of the exiled card will trigger when the token is put onto the battlefield. Any "as [this permanent] enters" or "[this permanent] enters with" abilities of the exiled card will also work.
- 1 January 2011If Prototype Portal has left the battlefield by the time its second ability resolves, you'll still put a token onto the battlefield that's a copy of the exiled card. On the other hand, if Prototype Portal is still on the battlefield at this time but there is no exiled card (because, perhaps, Riftsweeper's ability caused the exiled card to be put into its owner's library), no token is created.