Hamrah's $1.80 is simply a depreciation figure.
Someone, somewhere has to fork out the $36m to buy the 737-700. They will want some return on their outlay (whether a Bank, leasing company or Grandma). Interest charges on that outlay at 5% would be $1.8m per annum. (Gulp)
That's $150,000 per month - more than depreciation - $1250 per sector or $8.45 per passenger (I think Hamrah's $1.80 depreciation figure should be $7.51 per passenger).
Adding the two together gives an aircraft holding cost of $15.96 per passenger - roughly £9 per passenger sector. That seems high, so I guess the aircraft do more than 4 sectors a day (more like 8, which would halve the figure). Also, perhaps discounts are available on 737-700s!
I would be interested in anyone's view - are you out there, Beancounter?