I value them on a rolling scale based on new purchase price and 2 hydro tests, making a lifetime cost of approx £200 and the value dependant on its position in its lifetime less any remaining tests.
So £200 / 15 * remaining years of life, then less £25 for every 5 years remaining
Typically they sell second hand at around £80, so if it's before half way through it's life then you're getting a good deal at £80, much nearer the end and it's worth negotiating
Specific cylinder and regulator combinations could be worth more
Note that the ultralight cylinders such as safer and fuel will cost a bit more, and have a 10 year maximum life. Expect to pay more but benefit from a true lightweight cylinder
A new aluminium cylinder will be about £30
A new fibre cylinder with regulator about £150
I would not bother with a second hand aluminium unless it was very cheap, a second hand fibre approx £80 depending on specifics