Not sure I can help, but if the cylinders are in still in test inspection date interval and if you have original purchase receipts, then the rig would be worth much more. Also are the regulators single or dual stage? -- dual stage would be worth more. Without actually seeing it, its just a WAG on value.
Some cylinders have their capacity stamped around the neck by the distributor, but most don't. They are usually sized by their external dimensions.