Cheapest priced copy (Eldritch Moon): $0.49 at Card Kingdom, near mint, their price list as of .
Lupine Prototype 
Artifact Creature — Wolf Construct · 5/5 · Rare · Eldritch Moon · #197
This creature can't attack or block unless a player has no cards in hand.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $1.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Eldritch Moon | 197 | Rare | $0.49 | $0.49 |
| Innistrad Remastered | 267 | Uncommon | $0.49 | $0.49 |
| Innistrad Remastered | 444 | Uncommon | $0.79 | $0.79 |
| Eldritch Moon Promos | 197s | Rare | — | $1.29 |
2 rulings from Wizards of the Coast
- 13 July 2016Lupine Prototype’s restriction applies only at the moment that attackers and blockers are chosen. If a player puts a card into their hand after Lupine Prototype has attacked or blocked, it won’t be removed from combat.
- 13 July 2016If a creature can’t attack or block unless you return a permanent to your hand, and your hand is empty, you can attack or block with that creature and Lupine Prototype. This is because the legality of chosen attackers and blockers is checked before costs are paid.