[ok_craig]

The cost of a bitcoin transaction is not $50, it's about $0.06. Often times, such as for large transactions or moving old coins, you don't need to include a fee at all.

[gst]

The direct cost per transaction (= the mining fees paid by the person who initiated the transaction) is $0.06. The externalized cost per transaction (= the block reward divided by the number of transactions) is $50.

[ok_craig]

(1) That's not made apparent in the sentence (why leave it for a footnote?), and (2) you haven't explained why that number is actually meaningful.

Edit: I don't know much about this stuff so please correct me if I'm wrong but wouldn't the cost of the "externalized price per transaction" be (price_to_mine-block_reward)/number_of_txs. price_to_mine and block_reward would approach each other over time.

[clarkmoody]

Could you explain a little more clearly the "externalized" cost? I.e. who actually incurs the cost of the block reward?

I understand the equation of ([transaction fees] + [block reward])*[exchange rate] / [#transactions in block].

[asciimo]

This is not a "cost" because no one pays it. The 25BTC are created out of thin air as a reward to the miner. I don't see the relevance of this metric to your argument, but I hope that you will edify me.