The price for Download and retail are the same, because the retailer want it.
the analysis below is true ONLY for PC, for Consoles, there are other specific surprises
Of course you could imagine that, as a customer, you have the right to pay only for what things worth, and of course it is not true... We are the consumer, the lower step on the food chain.
So to make a summary of the situation, the download version cost less, of course, there is not "cost of goods"(that is 0.5-1USD) that's the box the manual.
There are no logistics (no truck to deliver boxes from warehouse to warehouse). the cost for delivery of games in term of bandwitdth is nothing, few cents for the people delivering you the game. the distribution cost retail vs web shop is something like 30-50% of the price on retail, less than 20 on digital, of course some portals take more, but they bring something more usually, or they sell so little that it doesn't worth the embarassment.
In the end you could pay your Digital version 30% less than the retail. BUT the retail is blackmailing the publisher and say : if you put it at 19.99 in digital and 29.99 retail, we'll put it too at this price, otherwise all people will buy digital over retail, and retail will sell ONLY to the the people that REALLY want a box, not enough people to actually reference and carry a product.
So if you want a box, and if publisher make boxes, they have to keep the price of the game more expensive for everyone. Of course many publishers are not interested into that, and stop to make boxes.
this is why, there will never be price differences for NEW games between digital and retail, after few weeks or months, you'll be able to observe divergence between retail and digital.