The way I think about Bitcoin is like this:
Bob wants to conduct a transaction with Alice using Bitcoin. Who can help Alice prove that she really owns and is giving Bob the number of Bitcoins that she says that she is?
Someone with a really fast computer. Luckily, Manny has one of those. Manny will gladly help Alice and Bob out, but only if his costs are paid for and he gets a little something for his effort. The proof takes energy, and energy has a cost. So does Manny's equipment. And, quite honestly, Manny would rather be on the beach than spending time purchasing, installing, and tending to that equipment. (not the mention the risk Manny is taking by owning all that expensive equipment in the first case). But he is rewarded/paid enough that he will run the proof.
The average cost of a bitcoin will tend toward the cost of mining a bitcoin (or earning a bitcoin in fees). It would be interesting to compare at this time given that the cost of a bitcoin is easily measurable in USD, EUR and the like. Cost of equipment and energy are relatively easy to know, which could make measuring real fiat labor costs relatively easy. Then again, it is difficult to know how many see this as a hobby (or are teenagers/young adults stealing electricity from their parents).