What's the deal with oil? If you look across the Bible, there are many instances where oil is used. The anointing of Saul and David; Jacob pouring oil over the rock where he had his dream of the staircase to heaven(?); Maria pouring oil on Jesus, etc. Was oil expensive back then or something?
The short answer is yes, oil was expensive. It was used in ceremonies both because it was often perfumed to have a pleasing aroma and because the financial sacrifice was an act of humility. 

You mentioned several good examples, but I figured I would offer you one more: Elisha providing for a widow (2 Kings 4:1-7). In this scenario, Elisha is able to provide the woman with enough oil that she can pay off all her late husband's debts, and she and her sons are able to live on the rest. 

Now, this passage doesn't explicitly state that oil was expensive, but a little common sense will tell us it can't have been cheap. Elisha sends the woman's sons for empty vessels, and not too few, so he was clearly preparing to give her a large volume of oil, but it's fair to guess that the number ended up being in the 20-50 range (obviously just an estimate) as opposed to in the hundreds or thousands. For that amount of oil to be enough to pay off debts and set multiple people up for life means that oil had value, far more than it does today (imagine how much olive oil you'd need to hoard to be set for life).