Cheapest priced copy (Mirage): $899.99 at Card Kingdom, near mint, their price list as of .
Lion's Eye Diamond 
Artifact · Rare · Mirage · #307
Discard your hand, Sacrifice this artifact: Add three mana of any one color. Activate only as an instant.
Legal inCommanderLegacy
Restricted inVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $899.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirage | 307 | Rare | $899.99 | — |
One ruling from Wizards of the Coast
- 4 October 2004The ability is a mana ability, so it is activated and resolves as a mana ability, but it can only be activated at times when you can cast an instant. Yes, this is a bit weird.