Francium is so rare that you would not be able to buy it. It is estimated that there is only 340 to 550 grams of this element in all of the Earth's crust at any given time. It is radioactive and has an extremely short half life. It's half life is so short that we have not been able to produce it in large enough quantities to even be able to determine its chemical properties. So, considering how little their is of the stuff, you really can't put a price on it.

1) "Francium (IPA: /ˈfrænsiÉ™m/), formerly known as eka-caesium and actinium K, is a chemical element that has the symbol Fr and atomic number 87. It has the lowest known electronegativity and is the second rarest naturally occurring element (after Astatine). Francium is a highly radioactive metal that decays into astatine, radium, and radon. As an alkali metal, it has one valence electron. Marguerite Perey discovered francium in 1939. Francium was the last element discovered in nature, rather than synthesized. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 is continually formed and continually decays. Perhaps an ounce exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount ever collected of any isotope was a cluster of 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1996." 2) "Isotope Half Life Fr-212 20.0 minutes Fr-221 4.8 minutes Fr-222 14.3 minutes Fr-223 21.8 minutes" Source: http://www.chemicalelements.com/elements/fr.html So, suppose you buy 128 mg of Fr-223, this is what you get after some time: - 21.8 mn: 64 mg left, - 43.6 mn: 32 mg left, - 65.4 mn: 16 mg left, - 87.2 mn: 8 mg left, ... - 174.4 mn: 1 mg left (after less than 3 hours!) So I would not buy any, if I had not a good reason to use it at once! 3) "Due to the small amounts produced and its short half-life, there are currently no uses for francium outside of basic scientific research." Source: http://education.jlab.org/itselemental/ele087.html 4) "A SUNY-Stony Brook team led by Luis A. Orozco and Gene D. Sprouse have developed a technique for successfully trapping thousands of atoms of francium, the heaviest alkali and the most unstable element among the first 1 03 of the periodic table. Less than an ounce of francium exists naturally on the Earth at any one time and the most stable isotope has a half-life of only 20 minutes. The Stony Brook researchers prepare francium-210 (half-life=3 minutes) at their accelerator and after reducing its energy by some fourteen orders of magnitude capture it into a magneto-optical trap. The trap is formed by six laser beams and a nonuniform magnetic field. The possibilities of carefully measuring the atomic properties of francium opens new horizons for the understanding of the atomic structure of a very heavy element. Studies of trapped francium can ultimately lead to high-precision measurements of a phenomenon known as parity nonconservation, which would then provide information on the weak force." Source: http://flux.aps.org/meetings/BAPSMAY96/vpr/aippr.html Further information: Scientists Trap Rarest Element — Francium: http://www.aps.org/publications/apsnews/199607/upload/jul96.pdf "They produced the francium in Stony Brook’s superconducting LINAC, and then succeeded in transferring the atoms into a magneto-optical trap for further careful study of the atom’s properties. Francium is the heaviest alkali atom, and holds special interest because its atomic properties can be calculated with high precision, says Sprouse. It can be used to study fundamental interactions, in particular the strength of the weak interaction between electrons and quarks in the atom, and the effects of the weak interaction in the nucleus. These effects have already been observed in cesium, but in francium, they are much stronger and should be easier to measure, says Sprouse. The Stony Brook accelerator has recently shut down, so Sprouse’s collaborators will carry on the francium work at TRIUMF, in Vancouver" Source: http://www.aps.org/publications/apsnews/200703/upload/032007-Vol-16-No-3.pdf So you need - at least the possibility to use - a particle accelerator.