[111] Jumpstart was designed to make it much easier to get into Magic by eliminating the deck-building but still providing some customization and randomness that comes with card acquisition and deck building
Now yes, Double Play is expensive for a Rampant Growth
Basically, you sacrifice the artifact, activate your merfolk commander to play it from your graveyard, trigger Mirran Spy to untap your commander, then rinse and repeat as many times as cards you have left in your deck
If you have a Commander that likes to smack people, consider Tenacious Underdog as a powerful enabler that draws you cards
So what are the approximate odds
Having first strike makes it easy for Altair Ibn-La'Ahad to survive combat, with so many ways to have him kill a blocking creature first before it deals damage to it