Until now I've always used PET soft drink bottles @ ~ 40g for 1.5L.
While I like the look of platypus flexible bottles they are expensive considering they are more fragile and break easier.
Back to the point, I found Aqua Essence 1.5L bottles that only weigh 22g. (Lighter even than platypus bottles)
While one bottle may not make much difference, I find that on some trips if I do not camp near a water source I want to have 4L of water at my camp (for tea/coffee/breakfast/washing up).

Sorry about this image, I can take another when I get home if you want.
I bought 10 at a sale for 10$ but retail price is like most other water bottles.
If you want to buy them they have a store location map on their website: http://www.aquaessence.com.au/
I am not affiliated with them in any way.
-icefest