If you shop games for a few years you see trends.
Nintendo games rarely go down more than 50% at any point in their life and generally hold at 100% for a while after launch.
Niche Japanese games split 50/50 for me with some dropping quick to 30% of initial price while others hold at 100% or even go to 120% due to limited stock.
The easiest drops to predict are mid major and major western releases. I knew Shadow of Mordor was going to be 50% off within 3 months of release, and it was over the holiday sales. By early to mid next year you'll be able to get it for $15-20 range.
I learned this on the PS3 when I bought Assassin's Creed for $60, Assassin's Creed II for $30, Brotherhood for $20 and Revelations for $10. Given how fast the entries come out if you don't mind being one behind easily find last year installment for sub $20.
My average purchase price for a physical PS3 game was roughly $20-25, meanwhile on 3DS it has been $30-35. I expect PS4 to follow this trend in coming years. Just look at near launch titles now to see how many have dropped to $20-30 range.
