Crabomination

The mana value of the back face of a double-faced card is the mana value of its front face. The mana value of a melded permanent is the sum of the mana values of its front faces. An artifact that's a copy of either has a mana value of 0.

You may sacrifice an artifact with mana value greater than or equal to the emerge cost. If you do, you'll pay only the colored mana component of the emerge cost.

You may sacrifice an artifact with a mana value of 0, such as a Food token, to cast Crabomination for its emerge cost. You'll just pay the full emerge cost with no reduction.

The spell you cast without paying its mana cost is cast during the resolution of Crabomination's triggered ability. Timing restrictions of that spell based on card type are ignored.

If you cast a spell for another cost "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, those must be paid to cast it.

Crabomination's triggered ability doesn't allow you to play land cards, and as such, any land cards exiled with Crabomination's triggered ability will remain in exile. If you choose not to cast any of the exiled nonland cards (either because you can't or don't want to), those cards will remain in exile as well.