Cheapest priced copy (March of the Machine Tokens): $0.35 at Card Kingdom, near mint, their price list as of .
Phyrexian Hydra 
Creature — Phyrexian Hydra · 7/7 · Rare · Mirrodin Besieged · #85
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
If damage would be dealt to this creature, prevent that damage. Put a -1/-1 counter on this creature for each 1 damage prevented this way.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
3 printings · $0.35 to $1.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Mirrodin Besieged | 85 | Rare | $1.79 | $0.99 |
| March of the Machine Tokens | 11 | Common | $0.35 | $0.49 |
| March of the Machine Tokens | 12 | Common | $0.39 | $0.49 |
2 rulings from Wizards of the Coast
- 1 June 2011If damage dealt to Phyrexian Hydra can’t be prevented (due to Leyline of Punishment, for example), it will be dealt as normal and won’t cause Phyrexian Hydra to put -1/-1 counters on itself.
- 1 June 2011If multiple damage prevention effects, including Phyrexian Hydra’s, are attempting to prevent damage that would be dealt to Phyrexian Hydra, its controller chooses which to apply.