I actually just wrote something about controllable and uncontrollable variables here: //business-finance.blurtit.com/3937494/uncontrollable-variables-are-not-really-constants-can-you-discuss - I did something similar in a module called 'Cost Accounting" about 2 years ago, didn't really enjoy it (I actually dropped out shortly after)!
Anyway, to answer your question, from the point of view of a purchaser, variables need to be assessed in terms of risk and predictive power.
A purchaser or consumer will try to work out what the likelihood of price increase/decrease is, and whether a purchase will represent a good return on investment.
He can do this by looking variables that affect pricing, and whether they are likely to change. Constant variables obviously won't, where as uncontrollable ones may - for better or worse.
This patterns can then be used by retailers or whoever to predict spending patterns and consumer/purchaser trends.
Here's some more detail on the whole process for you: http://www.academia.edu/2448406/The_effect_of_control_variables_when_using_Technology_Acceptance_Model_to_predict_consumers_intention_to_buy_online