The July 2009 issue of Aircraft Commerce had an article titled: "Narrowbody freighter selection for low utilization operations". The article gives the following figures for an express-package carrier:
Cash operating costs:
$5,900 for a 737-300F
$6,300 for a 737-400F
Maintenance costs per flight hour were ~$1,100 for both models.
Lease costs for both models were calculated at $2,500 per trip.
Overall trip costs:
$8,000 for a 737-300F
$8,700 for a 737-400F