With the great decrease in oil prices, has it reduced the cost of shingles, or can that be expected?

No, won't reduce the cost of shingles; it will take a couple of years for the reduction to show up. Guess you have to suck it up and have your roof done, the same as I did on southeastern Ontarioo for what, $3000. To last for 25 years ? I will be far gone by then.