Cheapest priced copy (Urza's Saga): $0.39 at Card Kingdom, near mint, their price list as of .
Veiled Sentry 
Enchantment · Uncommon · Urza's Saga · #109
When an opponent casts a spell, if this permanent is an enchantment, it becomes an Illusion creature with power and toughness each equal to that spell's mana value.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.39 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Saga | 109 | Uncommon | $0.39 | — |
4 rulings from Wizards of the Coast
- 4 October 2004If the spell is no longer on the stack when the triggered ability resolves, the mana value of the spell is still remembered.
- 4 October 2004It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 4 October 2004It changes into a creature even if the spell is countered.