Cheapest priced copy (Conflux): $0.35 at Card Kingdom, near mint, their price list as of .
Sludge Strider 
Artifact Creature — Insect · 3/3 · Uncommon · Conflux · #126
Whenever another artifact you control enters or leaves the battlefield, you may pay {1}. If you do, target player loses 1 life and you gain 1 life.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Conflux | 126 | Uncommon | $0.35 | $0.49 |
| Planechase | 95 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 1 February 2009If Sludge Strider and another artifact enter under your control at the same time, Sludge Strider's triggered ability will trigger. Similarly, if Sludge Strider and another artifact you control leave the battlefield at the same time, Sludge Strider's triggered ability will trigger.
- 1 February 2009If an artifact creature with 0 toughness enters under your control, Sludge Strider's ability will trigger a total of twice: Once when that creature enters, and once when it's put into a graveyard. Both abilities will be put on the stack after the state-based action check that puts the 0-toughness creature into the graveyard is finished.