I've received a quote from Andis Lau at HK Oxygen last year (his email is Andis.Lau at hko.linde.com). They quoted me HK$198 for a refill, but I assume that it doesn't include pickup/delivery (I believe delivery is about $28, or $53 for HK Island). They do have a plant in Tai Po where you can drop off your cylinder....
They also rent out cylinders for HK$295 (size G, whatever that means) plus $55 monthly rental charge, and a $1200 deposit.
Hope that helps....