The price of a product is related to its cost.
This is a logical assumption, but almost always false. It is far more commonly true that the price of a product is dictated by what consumers are willing to pay. If the PS4 version of game X sells for $59.99, it is more than likely that the Xbox1 version will sell for the same price, because people will perceive both versions as having the same value and be willing to pay the same amount for them both.
What Valve does with Steam prices is something different. New releases still cost roughly the same as traditional DVD copies bought in stores. Bargains start to appear on titles once they reach an age where a lot of stores stop stocking them. It is true you can get heavily discounted games on Steam, but this is totally unrelated to the lack of a used-game market. Somebody simply realized that a) A title nobody is selling makes no money and b) gamers will spend money they wouldn't have otherwise if they think they're getting a deal. Put a and b together and you have a recipe for profit. This also eliminates demand for used games. Why buy a skeezy dog-chewed box when you can get a steam-download for the same price or lower?
Don't get me wrong. If MS builds a curated steam-style store for the Xbox it will undoubtedly be a great thing for many (although not all) gamers. However, lower prices on new titles will not be one of the benefits this move brings. This is really just a grab for dollars that currently go to the used market, and it will likely work.