If 24 hours have passed since you purchased the ticket, then you will need to know the refund / cancellation policy on your ticket. If you book the cheapest ticket, then it will usually be non-refundable. Just because you booked an economy ticket, it does not always mean that it will be non-refundable because there are different classes within each fare class. This will get a bit technical, but there are fares such as Discount Economy Class, Discount 7 Days, Full Economy Fare, and such that all have different cancellation rules. Unless you are a travel agent or use points to book regularly, you may not even realize that the "Fare Class Y" on your ticket actually means something.
As I had mentioned before, if you are unsure and would really like to see if the airline will do anything regarding the price drop, just pick up the phone and ask the airline!