Cheapest priced copy (Weatherlight): $20.99 at Card Kingdom, near mint, their price list as of .
Mana Web 
Artifact · Rare · Weatherlight · #152
Whenever a land an opponent controls is tapped for mana, tap all lands that player controls that could produce any type of mana that land could produce.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $20.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Weatherlight | 152 | Rare | $20.99 | — |
3 rulings from Wizards of the Coast
- 1 April 2008The lands that are tapped by Mana Web’s ability don’t produce mana as a result.
- 1 April 2008When a land an opponent controls is tapped for mana, this card checks which types of mana (white, blue, black, red, green, or colorless) that land could produce using any of its mana abilities, not merely the one the player just used. Then it checks which lands that player controls have abilities that could produce any of those types of mana. Then it taps those lands.
- 1 April 2008The opponent can tap their lands for mana in response to Mana Web’s ability triggering.