Iron Drill

Iron Drill6 GRAM in game (≈600 CROWN at the presale rate), 3 GRAM in the presale

Adds +4 mining power. The power of every drill you own adds up, so a new drill does not replace the old one, it stacks on top.

Shards per hour from this drill = 4 × 300 × buildings × boosts × depth

At current competition that is about 25.93 CROWN per day, paying back in roughly 23.10 days.