In some conversion tables (I use tradtionaloven.com for my conversions for baking), 1 tablespon of sugar is approximately 1/2 oz, and 4 tablespoons is 2 oz. which matches the number I came up with in my information above from other sources, which should target the 4 volumes of co2 per gallon.
Generally speaking though, any amount of sugar will provide some fizz. The question always is how carbonated does a person like it. (I like my root beer REALLY carbonated. Then the big burps of course. It's required in the handbook to have really good burping rootbeer, isn't it?)
As for truvia, truvia is really sweet so the inital cost is for an amount that should give some good usage. We'll have to find the costbreakdown to cost to equal sweetness. Alternatively, you can just grow your own stevia. Mine always gets aphids though so I've not used it in anything.