Quote from: tymothy on August 17, 2013, 08:16:31 PM

I paid cash for a BFL 60 gh/s unit at a pretty heavy markup in USD. While I don't think the unit will ever generate the number of bitcoins I could have bought by investing that money directly in bitcoins instead of the unit, I do expect the USD value of my BTC will eventually exceed what I paid.

So you will lose funds. You could have bought BTC instead.

If you buy a miner (a device which produces BTC) it doesn't matter what currency you spend it has an equivelent price in BTC.

If the price if 1 BTC and over the life of the miner (becomes becoming obsolete and going dark) it produces 0.99 BTC net (after electrical cost) then you lost. You could have instead just held or bought 1 BTC and had 1 BTC. The future exchange rate can't improve your return.

Simple version:
you always have the option of simply buying Bitcoins so for a miner to be more profitable it has to return MORE bitcoins than it purchase price.