How would you do a calculation on that (energy from collapsing hole through spacetime)? I've googled to get somewhere but I can't find anything useful.

When taking General Relativity I did that calculation as part of the take-home exam. It's mostly a matter of calculating the curvature tensor of a wormhole and finding that it is freaky, so the energy-momentum tensor that would satisfy the Einstein equation would need to include negative energy and/or momentum (the Einstein equation basically says curvature tensor = energy tensor(1)). Then exactly how the thing would unravel is a matter of computation, but unravel it would.

Or one could presumably find a form of matter that has negative energy density and start building wormholes.

1) The energy tensor has energy as the time diagonal value, and momentum as the space diagonal values.

Protip: googling for the energy required to create such a wormhole is a better solution. That lands you on this BBC story[1] where they've interviewed well-known physicists, and state that

"As a very rough approximation, you would need the energy the sun produces over 100 million years to make a wormhole about the size of a grapefruit."

[1] http://www.bbc.com/future/story/20140326-will-we-ever-travel...

So approximately the energy equivalent of the mass of 2 earths.

Seems doable.

Ianahep, but maybe work backwards from gravity vs spacetime equations? I'm assuming gravity is the exchange mechanism that drives this, so if you needed x gravitational field to produce y displacement, then given y displacement you have some energy equivalent of x in mass?

A starting point is "stress-energy tensor" as a keyword to search.