Cheapest priced copy (Aether Revolt): $0.35 at Card Kingdom, near mint, their price list as of .
Siege Modification 
Enchantment — Aura · Uncommon · Aether Revolt · #99
Enchant creature or Vehicle
As long as enchanted permanent is a Vehicle, it's a creature in addition to its other types.
Enchanted creature gets +3/+0 and has first strike.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Aether Revolt | 99 | Uncommon | $0.35 | $0.49 |
| The List | AER-99 | Uncommon | $0.35 | — |
One ruling from Wizards of the Coast
- 9 February 2017If Siege Modification becomes unattached from a Vehicle that’s attacking or blocking, that Vehicle will be removed from combat unless another effect (such as its crew ability) is also making it a creature.