How to cite prices in academia?
There are two, as I see it very different questions you pose. as for the aptness of the sources, the Shea and Morgan paper is a publication that can be referenced because it is traceable and likely to be remaining for a long time in unaltered state. It is also published by a formal body.

When it comes to citing prices the question is perhaps trickier. Not knowing the market for helium-3 I do not know where the prices are determined but usually there is some form of exchange that determines prices based on the market and this makes prices shift quickly. In such cases, one needs to find a source where the market swings are documented and stored so that one can cite a price at a specific time in a time series that people can revisit if they wish to check the correctness of your assessments. This type of referencing is common in economics when discussing price variations, trends etc.