Ann: Binding offtake and funding agreement, page-142

I think you are off the planet in looking at the price of Rubidium metal. Almost all the price is from the cost of refining this metal. Rubidium is the next alkalai metal to caesium in the perodic table and just above. This means that it has similar properties to caesium. I imagine that it could be substituted in the light sensitive applications. I know that the formate is used in oil drilling. The atomic weight of Rubidium is about two thirds of that of Caesium so the formate would be less valuable than that of caesium, but still very valuable for blending and cost reduction in applications where lower density is suitable. Potassium formate is already used as a cheap additive where lower density is acceptable. The atomic weight of Potassium is less than half that of Rubidium.