Stevia to Sugar Ratio

I have just found this amazing herb and plan to use it as a diet supplement. I have diabetes 2 and avoid sugar. I make a cocoa mix using Splenda, 2 1/2 cups. If I use all stevia I'll break the bank. I need a ratio of stevia vs sugar. Maybe I can substitute for a portion of the other sweetener? I paid $1.28 per ounce. Is there a better price anywhere? Thank you for your help, this is a great site.
FYI: I found reference to this in the periodical "Bottom Line".

If you purchased pure white powder extract that would be a terrific price. But I suspect that you bought a product diluted with a carrier such as maltodextrin. The maltodextrin is added to make the powder easier to measure.
Results vary depending on the product, but a typical serving of a typical stevia and dextrin mixture would be 1/4 teaspoon. On the other hand, 1/4 teaspoon of the pure extract without additives would be equivalent to a cup of sugar. Stevia is also sold as a green powdered leaf; in this case 1-1/2 to 2 tablespoons are equivalent to a cup of sugar.
To save money you can buy the pure white extract and dilute it yourself with maltodextrin. Richters offers pure white extract in a 25 gram (approx. 1 ounce) size and in a kilogram size (2.2 pounds). For details see: www.richters.com/Web_store/web_store.cgi?product=X6030