Unfortunately, quasi-price drops don't work as well as true drops.
Most consumers prefer a cheaper system as opposed to throwing in a game, because the average consumer feels that it's more worthwhile to pay less for the system, and then choose what they want, rather than be told by someone else what to get.
Now, some systems (namely Nintendo systems) don't follow this trend. In the US, most major Nintendo systems have been bundled with a game (Super Mario Bros/Duck Hunt, Wii Sports, ect), and it's worked. However, they did this from the get-go, rather than just throw it in at some point.
Also, what worked more for Sony. The fact that the model went to $500 instead of $600, or the fact it was a better buy? Most consumers only know of the $600 model. So getting it for $500 is a deal, thus the increased sales.