Cheapest priced copy (Future Sight): $0.35 at Card Kingdom, near mint, their price list as of .
Logic Knot 
Instant · Common · Future Sight · #52
Delve (Each card you exile from your graveyard while casting this spell pays for {1}.)
Counter target spell unless its controller pays {X}.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
3 printings · $0.35 to $7.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Future Sight | 52 | Common | $0.35 | $7.99 |
| Modern Masters | 51 | Common | $0.35 | $0.49 |
| Time Spiral Remastered | 73 | Common | $0.35 | $0.99 |
4 rulings from Wizards of the Coast
- 19 March 2021Because delve isn't an alternative cost, it can be used in conjunction with alternative costs, such as flashback. It can also be used to pay for additional costs that include generic mana.
- 19 March 2021To cast Logic Knot, you choose a value for X, determine the total cost, then pay the total cost with some combination of mana and exiling cards.
- 19 March 2021Delve doesn't change a spell's mana cost or mana value. For example, Treasure Cruise's mana value is 8 even if you exiled three cards to cast it.
- 19 March 2021You can exile cards to pay only for generic mana, and you can't exile more cards than the generic mana requirement of a spell with delve. For example, you can't exile more than seven cards from your graveyard to cast Treasure Cruise unless an effect has increased its cost.