Re Minimimising weight of checked in "suitcase":
In probably everywhere I've been you can buy large strong cheap (choose any 3) polypropylene (I think) bags which fold flat with a very small volume, are strong enough to resist airline handling and which cost very little (see photo below. They have a zip top, come usually in pastel stripes or tacky tartan or a generally ikky colour.
I and my family use these as what I term "Ocean Jumping Bags".
They are light enough and compact enough that that can be folded up and kept until the next ocean-hop arrives - but also cheap enough and available enough that they can be discarded and replaced when required.
In the absence of these life savers a piece of plastic tarpaulin plus parcel tape would be almost as useful, but lacks the zip and more easily sealed nature of the bag.
These bags can either be packed with all checked-in material and no other bag, or smaller bags & boxes and loose material can be placed in them.
They are usually of larger volume than required and, rather than committing a large floppy bag to the baggage handlers' tender care, I usually fold them into a compact bundle and then wrap them in parcel tape. The handles are sewn on and will tear off without much effort, but the actual bags are awesome strong.
As the bags offer little or no protection against impact I put more delicate material in the middle wrapped around with clothes (and sometimes extra packing if required) and / or with rigid less breakable objects and then wrap with tape. Such an arrangement has never been torn open in transit when I've used it, and would probably survive any conceivable treatment it would receive
OCEAN JUMPING BAGS
Three samples - various sizes.
The largest is
700mm square by 250mm thick and
weighs 210 grams (7 ounces) and
120+ litres capacity and
cost about $US5 retail late at night (In Australia, but relatively cheap anywhere).
Add a bit more weight for tape if desired.
Middle can be thicker if desired just by packing more in there.
Coke box for sizing. Tasteful editing to meet domestic censor's requirements.
That bag has more volume (120 litres + ) than the all except the very largest suitcase you'll be liable to find. (The large bag was bought in Australia and brought two backpacks back to NZ. The medium one was bought somewhere in Asia and brought now unknown sundries to NZ.)
Larger version of image here, fwiw
Added October 2015: I just visited Australia for two weeks. On the return flight I ended up on a China-Airlines flight. Reading the fine print showed the checked baggage allowance was 20 kg (unlike the now usual 23 kg) and they specifically said that bags combined together either by taping/wrapping or with several bags inside a large outer bag were unacceptable - the only time I've ever seen either of those stipulations.
In the event - at checkin we were told that the checked allowance was 25 kg and while the person on the counter was aware of the "not taped together" requirement it was clear that it WOULD have been acceptable if done well - they appeared to be trying to avoid a multi part mess thrown together badly so that it may come apart en route Given hjow baggage handlers sometimes throw bags around extremely roughly (based on numerous reports and my personal observations on a number of occasions) this seems a reasonable requirement.