Vanilla beans are expensive because they are a commodity and like all commodities, price fluctuates with supply and demand. When demand goes up, so does price. It is also the second most labor-intensive spice to cultivate behind only saffron. Each flower needs to be hand pollinated and harvested then cured over a series of weeks or even months. Finally, frequent natural disasters such as floods or cyclones in vanilla-growing regions can devastate crops and push up the price. These and many other reasons are why vanilla is so expensive. Time magazine recently published a great article on the subject.