Cheapest I've seen for the pure extract is about $7.50 at Whole Foods [in Coral Springs -- near my parents' place.] I can get the KAL Brand for $9.99 around here. The Green bottle [Sweet Leaf Brand] is about $12-13 for 0.9 oz. Both are very good quality.

1 oz. of pure stevia extract has the sweetening power of almost 3 pounds of pure cane sugar. 1 Bottle should be sufficient to sweeten 200-8 oz. Beverages. So, that little bit goes a long way. I use 1 tsp in place of 1 cup of Sugar/Splenda [and that is the basis of my figures.] I found that conversion in "Sugar-Free Cooking w. Stevia."