Cheapest priced copy (Shadows over Innistrad): $0.49 at Card Kingdom, near mint, their price list as of .
Brain in a Jar 
Artifact · Rare · Shadows over Innistrad · #252
{1}, {T}: Put a charge counter on this artifact, then you may cast an instant or sorcery spell with mana value equal to the number of charge counters on this artifact from your hand without paying its mana cost.
{3}, {T}, Remove X charge counters from this artifact: Scry X.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 to $2.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Shadows over Innistrad Promos | 252s | Rare | — | $2.29 |
| Shadows over Innistrad | 252 | Rare | $0.49 | $0.79 |
4 rulings from Wizards of the Coast
- 8 April 2016When resolving the first ability of Brain in a Jar, the newly-placed charge counter will be counted when determining what spells you may cast. No player may take any action between you placing the counter and choosing which spell to cast.
- 8 April 2016If you cast a card “without paying its mana cost,” you can’t choose to cast it for any alternative costs, such as awaken costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, such as that of Lightning Axe, you must pay those to cast the card.
- 8 April 2016If Brain in a Jar leaves the battlefield before its first ability resolves, use the number of counters on it at the moment it left to determine what spell you may cast. That number won’t change because you can’t put a new counter on Brain in a Jar.
- 8 April 2016If the card has {X} in its mana cost, you must choose 0 as the value of X.