For a beer brewed at 20c, 19 litres in a 19 litre corny, 2 litre pop bottle and wanting 2.3 vols of carbonation use 127.7g of sugar and the water in the bottle will get to 3.76% alcohol. I just use a spoon of bread yeast and a half spoon of nutrient. Throw in a load of trub instead if you like.
If it's cost you're thinking of then that's really not the point of doing it. The sugar will cost 8.3p let's say with the yeast and nutrient 15p total. To force carb with a pub gas bottle you'd use 54g of gas - about 16p.