Cheapest priced copy (March of the Machine): $0.49 at Card Kingdom, near mint, their price list as of .
Zephyr Singer 
Creature — Siren Pirate · 3/4 · Rare · March of the Machine · #355
Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Flying, vigilance
When this creature enters, put a flying counter on each creature that convoked it.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine | 86 | Rare | $0.49 | $0.49 |
| March of the Machine | 355 | Rare | $0.49 | $0.49 |
| March of the Machine Promos | 86p | Rare | $0.49 | $0.79 |
| March of the Machine Promos | 86s | Rare | — | $1.49 |
7 rulings from Wizards of the Coast
- 14 April 2023You can't tap more creatures to convoke Zephyr Singer than is necessary to pay for the spell. In most cases, this means four creatures. However, if there are any additional costs to cast Zephyr Singer, you may use convoke to pay those additional costs as well.
- 12 January 2024When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.
- 12 January 2024Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.
- 12 January 2024You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.
- 12 January 2024Tapping a multicolored creature using convoke will pay for {1} or one mana of your choice of any of that creature's colors.
- 12 January 2024If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.
- 12 January 2024Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.