In the past couple of years, I've never found a digital download to be cheaper then the physical copy of the game. It most likely will never be, since doing a digital download through a sole proprietor like Playstation Network or Xbox LIVE, the proceeds go directly to them and their usage of the rights. Steam is a massive third party type of deal so they are allowed to sell them at constant different rates, but not the same way on Consoles.
As I stated earlier, the entire reason we're still using discs is solely because of the SIZE of the content and the lack of high-speed coverage. It's common sense when you think about it. Things like Microsoft Office, and other programs that are under a few gigs have almost completely gotten rid of disc usage because even on low end DSL it can still be downloaded fairly quick. However, someone who doesn't have 30 to 50+ MB/s download speed, it'll take far to long for the average consumer to want to wait around for a digital download.