I figure each bitcoin transaction is currently costing about $US 0.0003 (three-hundredths of a cent)
Here's the back-of-the-envelope calculation I used to get to that number:
Transaction size: ~300 bytes.
Disk space cost: 0.05 $US per gigabyte (about $100 for a 2TB hard drive these days)
Bandwidth cost: 0.02 $US per gigabyte
CPU cost: 0 (bad assumption-- anybody want to try to come up with a better estimate?)
Size of bitcoin network: ~10,000 nodes
Each transaction is currently sent across the network twice (once on its own, and once when it becomes part of a block). It is stored on disk once. So disk+bandwidth cost for one machine is:
(0.05+0.02+0.02)*(300/1000000000)
... times 10,000 nodes gives $0.00027 per transaction.
Please check my work-- I usually screw up and add or drop an extra zero when doing these types of calculations.