A couple of weeks ago I bought a plane ticket in Expedia, from YVR to EZE, that cost me 1700 CAD. It's non-refundable and non-transferable. I was under the assumption that the earliest you buy the ticket, the cheaper it will be.

Yesterday, however, I went back to the website where I bought it and to my surprise, the same ticket is now 1000 CAD (same exact flights). I'm furious but I don't know if there is anything I can do. 

If there's nothing I can do, can I prevent something like this from happening in the future?