It all depends on condition and length of the test.
I paid £85 for 300 (4500 psi) bar 12 (12 is as big as 300 bar go) but had to collect it from Cornwall ,I was dropping daughter of at Falmouth uni so that worked out ok.
So around £80-100 would be a fair price they are expensive new.
It's best to have a long test ,as if it fails it gets cut in half.
Tests cost around £35-40.
Saying that though I've never had a diving bottle fail a test yet.
232 (3000 psi) 15 litre bottles are easier to get and cheaper as most learner divers start with them.(£75 for a good one in test)
You need a dive/ Airgun /site/ shop nearby to fill it for you fills are £3-5 .
Air gun web sell them second hand but usually 3 or7 litre ones are more popular with air gunners .
http://www.airgunbbs.com/