Isn't it more proper to say that there's cost? There's no "hidden value line", it just costs $X to mine gold, it costs $Y to make a house, it costs a frequently variable amount of money to mine a bitcoin.

But, the mining and acquisition costs in bitcoin depend (although not solely) on the demand. Difficulty does change according to the demand. Mining bitcoin isn't the same as mining gold, and therefore not a good example of the "minimum price", which is, as said, the cost.