The retro gaming scene has been around for a long time now, and the prices of systems/games tend to follow a certain curve:

1 - Start out high when new and drop.
2 - Bottom out (from what I've noticed, this is about when the games are 10 years old)
3 - Appreciate due to increasing scarcity and retro gamer appeal. This peaks around 20 year games.
4 - Depreciate once again and level out. This tends to happen when the game is 25 years old or more.

So that puts PS4/Xbone/Wii U (8th gen) firmly in Step 1 with much of the 7th gen still in Step 1. The 6th gen is firmly in Step 2 from what I've noticed, I got 100 PS2 games (disc only) for 20 bucks a year and a half ago. 5th gen is going into Step 3 (it was FIRMLY in Step 2 in 2008, when I got 63 4th and 5th gen games for just $17).

So it seems the 6th generation is at its cheapest. Good idea to focus collecting on it?