That's the point. Steam games should be cheaper because they make more money off of them then disk games.
You forget one thing though: games don't fit the traditional market schema simply because there is a minuscule variable cost. Digitally distributed game prices are completely decoupled from the costs which went into making the game; it's simply a question of 'what combination of price point, price reductions, and sales will make the most money.' You aren't paying based on dev costs or marketing costs, you are paying based on a schema in which they have attempted to guesstimate what will maximize revenue.
Which, as it turns out, has a marketing strategy similar to the following:
1. Release at full price; this is the price at which real fans or those looking forward to the game will buy it (maximize initial revenue)
2. Promotional sale/s some time afterwards, moderate discount (long enough afterwards for those who purchased in group 1 to feel like their earlier purchase was worth buying at full price)
3. Drop price point over time depending on number of people still buying the game with occasional extreme sales (for example, Oblivion for $5) in order to bring in the last bits of revenue and potentially generate new fans for future releases
That's pretty much how you get the most sales and the most revenue from a game, and I don't see that changing any time soon. This from a 4th year Game Dev major who has had to present business cases for game design pitches in class.
http://www.rockpapershotgun.com/2011/10/24/less-is-more-gabe-newell-on-game-pricing/