I think the cost per transaction will drop to more reasonable levels as the block reward drops.

As block rewards become trivial and then disappear, we'll see something interesting with the fees and hashrate, I think: With only transaction fees paying you, it's no longer worthwhile to mine when there are little to no transactions for you to claim. Barring other factors (e.g. thermal cycling could be harmful enough to the hardware to negate the benefit) then, logical, self-serving miners who pay for their own electricity/cooling would take the strategy of "when there's over x fees, it's worth it for me to mine" and have their machines automatically start and stop accordingly. This will drastically change the "block every 10 minutes" scheme we've got now, and make the hash rate harder to measure. 51% attacks would be much easier during a slower time (when most Bitcoiners are asleep, I suppose).