How much CO2 does it take to carbonate water using a household Soda Siphon? A soda siphon seems to be the de facto device for carbonating water at home.  

A search of reviews and comments on the consumption and cost of CO2 yields the following results:


70 cents or one charger per liter (Amazon Review)
20 cents per liter (see lotusmoss' blog comment)
50 cents per liter (see mbknight's blog comment)


The instructions for this popular iSi soda siphon seem consistent with an entire charger being required per liter of water, which works out to $0.90 per liter assuming you buy the recommended brand-name chargers.

So, the cost claimed seems to range from $0.20 to $0.90 per liter.  Since store-bought carbonated water typically costs somewhere in between, it's not obvious whether this device makes economical sense.  (I'm sure there are other advantages, but that's a different topic.)

My questions are as follows:


Does a charger like this really only yield 1 liter of carbonated water?
Is there a way use a single charger to yield more than 1 liter?
Is there an alternative to the iSi-style soda siphon that uses some other source of compressed CO2 that might be more economical?
Has anyone reproduced the $0.20 per liter figure claimed by lotusmoss?  If so, how was it acheived?
I own a 1-litre soda siphon like the iSi Siphon you pointed out. Some friends have a higher-capacity soda maker, I think from Sodastream.  To answer your questions:


Yes, the charger is single-use, and it carbonates only as much water as is in the bottle. My soda siphon holds about 1 litre of water. If I put in less water, then once the bottle runs dry I vent spare carbon dioxide uselessly into the air. If I put in more water, the carbon dioxide runs out and the remaining water won't leave the bottle.
Using the standard siphon product, there's no easy way to have a single charger yield more than one 1 litre. The charger is designed to deliver all its gas at one time; it doesn't reseal. 
Consider the Sodastream product line if you want a step up from the 1-litre soda siphon with disposable chargers. Sodastream uses larger bottles of gas, which they describe as "60L". I think they mean that each bottle of gas carbonates 60 litres of water.  
Sodastream's FAQ, What will it cost me to make a bottle of soda or seltzer? claims,



  After your initial purchase of a home soda maker package, you'll enjoy seltzer and sparkling water in reusable 1-liter PET bottle for just [Canadian]$.30 per litre and flavoured sodas for about $.88 per litre, before any applicable shipping charges....


From their wording, it's pretty clear to me that the $0.20-$0.30/litre claim is purely the marginal cost of carbonating water. It doesn't include amortising the purchase cost of the equipment, and likely is based on the cheapest way to refill the carbon dioxide bottles (exchanging filled bottles for empty by mail), and likely doesn't include shipping costs or taxes for the refills. It may be that one can drive the cost per litre down by adjusting the amount of water in the soda bottle, as well.