Francium is a very rare element. Some estimates suggest it exists in the Earth’s crust in quantities less than 30 grams, or about one ounce. It is extremely radioactive and has a half-life of just 22 minutes. This means that if you could somehow gather every atom of it on the planet, it would disappear almost instantly.
The only way to get a decent sample of francium is through the decay of other elements. For example, thorium and uranium naturally produce it through their own radioactive decay chains. Francium is very unstable and cannot be isolated in bulk. Even the largest amount ever observed (a cluster of 3,000 atoms) would vaporize almost immediately.
This is one of the reasons why it is so expensive. The other reason is that it has no practical use. There is no chemistry that can be done with it and, if you dropped a francium atom in water, it would react violently, liberating hydrogen gas.
Physicists have been trying to find new uses for the element. Recently, scientists at the State University of New York at Stony Brook have been working on a method to trap a collection of francium atoms in the middle of a magnetic field. They hope that this will allow them to perform measurements and learn more about the atomic properties of this very rare metal. But, for now, the only practical use of francium is to make other radioactive materials that are used in medical treatment and nuclear energy.