Based on context, you seem to be referring to gas when you say 12 million "ether gwei" (the block limit being 12 million gas, lately). But gas is its own unit, not measured in ether (or in wei, the smallest atomic unit of ether).

The reason the math still works is that 12 million gas priced at 1 gwei/gas would cost 12 million gwei, which is .012 ether. (Since one ether is defined as a billion billion wei) ... Maybe this was all what you meant, but it was hard for me to decipher.