How much CO2 does it take to carbonate water using a household Soda Siphon?
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.