Starbucks' Tall (small), Grande (medium) and Venti (large) Americanos (espresso and hot water) contain two, three and four shots (servings) of espresso, respectively.
One shot of espresso requires approximately of coffee.
Assuming is correct, there are 4, 000 coffee beans needed to make one pound of coffee.
There are approximately in a pound and, therefore, 8.81 beans per gram. Therefore, there are 61 and 70 beans to a shot, but lets just take an average and say it is around 66 beans to a shot.
Doing a little math, there number of beans needed for Starbucks' Tall, Grande and Venti Americanos are aproximately 132, 198 and 264, respectively.
Prices for Starbucks' drinks vary throughout the world, but in my neighborhood (Southern California), it would be $2.15, $2.55 and $2.95 for the three sizes.
This gives you a price per bean of $0.016, $.0.013 and $0.011 for the three sizes, with the Venti size giving you the most bang for the bean (i.e. lowest cost per bean).