3 - 4 years? Unsually the price drops are much much sooner.
I got R&C Rift Apart just five weeks after release for ~€50:
Yup, as I state I got demon souls for 200 BRL (launch price was 350), and the game isn't even 2 years old yet. I meant 3 to 4 years because it's already enough time to have a decently sized host of games costing less than 70 USD
I believe after 4 years every Sony first party will be already down to 30 USD