I just used the Canon WP-DC34 in Similans last week, it cost me like SGD 319.00. They have limited stock so you need to check with the Canon office in Vivocity if they have the casing. Just google for canon singapore contacts, you can just give them a call. Also saw some casings available at Cathay, etc but not sure how much they cost there.
Slightly off topic but just to add on for your reference:
It's extremely bouyant and the casing fogs up fast so if you are going with the casing option do look at buying weights and putting more silica packets in the casing. I was extremely irritated with the diffuser which was very loosely attached to the casing and will keep coming out very easily. I'm not sure if you can attach a strobe but definitely go for that if you can.=)
Also a note for this casing is that you have no access to the thumbwheel functions. (so i ended up having to preset my lens/shutter settings before going under)
Anyways having said all that, was happy overall and for casual u/w photographers this should be a sufficient setup but if you have money definitely look at getting patima/ikelite housing for the G11 as well as strobes. (which is what Im looking at now! must save up the moolah!)=p
Hope that helps.