Cheapest priced copy (Time Spiral Timeshifted): $0.35 at Card Kingdom, near mint, their price list as of .
Spined Sliver 
Creature — Sliver · 2/2 · Uncommon · Stronghold · #130
Whenever a Sliver becomes blocked, that Sliver gets +1/+1 until end of turn for each creature blocking it.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.35 to $22.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Premium Deck Series: Slivers | 14 | Uncommon | — | $0.49 |
| Stronghold | 130 | Uncommon | $0.39 | — |
| Time Spiral Timeshifted | 101 | Special | $0.35 | $22.99 |
3 rulings from Wizards of the Coast
- 4 October 2004If two Spined Slivers are on the battlefield then each blocked sliver gets +2/+2 for each creature blocking it.
- 1 July 2013Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit.
- 1 July 2013If the creature type of a Sliver changes so it’s no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.