Current mining:
Difficulty: 7,672,999

62,172.58 GH/s

969.89 megawatt hours

$145,483.85 / day

3600 bitcoins created / day

$145,483 / 3600 = $40.41

If you want fundamentals. You cannot charge less for a product than it takes to produce.

But for every product you buy you have to consider:

cost of production + cost of distribution + cost of retail + cost of marketing  at the very least.

Price rarely dips below difficulty for this reason.