I sold a pair of gold LP cylinders last year for £35 plus postage - no leaks, years out of date but no gauges on the end-old style.
I also bought some with gauges and 5 years I n date - simplistically, the agreed sale price was the new cost less £30 residual value as if they were out of date, plus the remainder of the new cost divided by 10 years then multiplied by the number of years left. So say £120 new cost less £30 =£90/10 years =£9/yr, x 5 years left=£45 plus £30 residual=£75
Hope that helps!