Cheapest priced copy (The Brothers' War Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Myr Battlesphere 
Artifact Creature — Myr Construct · 4/7 · Rare · Phyrexia: All Will Be One Commander · #138
When this creature enters, create four 1/1 colorless Myr artifact creature tokens.
Whenever this creature attacks, you may tap X untapped Myr you control. If you do, this creature gets +X/+0 until end of turn and deals X damage to the player or planeswalker it's attacking.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
16 printings · $0.49 to $16.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Double Masters | 276 | Rare | $0.59 | $2.99 |
| The Brothers' War Commander | 151 | Rare | $0.49 | — |
| Commander 2013 | 247 | Rare | $0.79 | — |
| Commander 2014 | 254 | Rare | $0.49 | — |
| Commander 2016 | 263 | Rare | $0.69 | — |
| Commander 2018 | 212 | Rare | $0.49 | — |
| Commander 2021 | 253 | Rare | $0.49 | — |
| Commander Anthology Volume II | 202 | Rare | $0.59 | — |
| Duel Decks: Elves vs. Inventors | 57 | Rare | $0.69 | — |
| Foundations Commander | 273 | Rare | $0.49 | — |
| Modern Horizons 3 Commander | 301 | Rare | $0.49 | $1.49 |
| March of the Machine Commander | 366 | Rare | $0.49 | — |
| Phyrexia: All Will Be One Commander | 138 | Rare | $0.59 | — |
| Secret Lair Drop (Egg Hammer) | 2097 | Rare | $1.49 | $1.99 |
| Scars of Mirrodin | 180 | Rare | $0.99 | $16.99 |
| Tarkir: Dragonstorm Commander | 322 | Rare | $0.49 | — |
3 rulings from Wizards of the Coast
- 7 August 2020You choose the value for X as the last ability resolves. You can't choose a value for X that's greater than the number of untapped Myr you control.
- 7 August 2020As the last ability resolves, you can tap untapped Myr you control even if Myr Battlesphere is no longer on the battlefield by then. If that has happened, Myr Battlesphere won't be able to get the +X/+0 bonus, but it will still deal X damage to the appropriate player or planeswalker.
- 7 August 2020You can tap any untapped Myr you control as the last ability resolves, not just the Myr tokens you created with the first ability. This includes Myr that haven't been under your control since your most recent turn began.