by what measure? 100lb of helium is exactly the same by weight as a 100lb of propane. By volume, in a cylinder, it is approximately the same (I believe).

By number of atoms, it is much more (a factor of 10 or so).

Weight is the same, but because the number of atoms is so much higher isn't the pressure a lot higher? PV=nRT and all that?

I presume the leakage loss in storage also raises the price of Helium. People buy propane just-in-case knowing they can store it forever while helium is a "buy it before you use it" kind of thing.