I guess this article is mostly relevant in the US.
Here in EU, or at least Sweden, Steam rarely have affordable prices compared to what you get in stores, except for on occasional sales. Very often they set the same number at the price tag for US and EU, even though US$ and ? are not equal in worth 20? is more expensive than $20. Steam has gotten better at this recently, but it still seems to happen, and very rarely do they lower prices of older products.
I have bought stuff on steam occasionally, but it's very rare I see something a lot cheaper than if I had bought a physical copy at a store.
A couple of examples, I bought Borderlands in store for 40? a week after it was released. 5 months later, and it's still priced 49.99? on Steam.
Section 8 is priced between 10-20? in stores, and 39.99? on Steam, Red Alert 3 cost well below 10? in most stores, but is priced 24.99? on Steam.
OK, those were 3rd party titles, so maybe Valve/Steam has no saying on what is reasonable pricing for those, let's look at some Valve titles instead, once you buy the physical copies you still get all the steam benefits from those, since they are activated on steam:
The Orange Box is priced 29.99? on Steam, and cost about 20-25? in stores,
Half-Life 1 is priced 9.99? on Steam, that's what the entire Half-Life Anthology cost in stores (14.99? on Steam), and then they still charge separately for Team Fortress Classic that came free with the physical half-life copy 10 years ago.
Left 4 Dead (1) is still priced at 29.99?, while it's usually priced 15-20? even in more expensive stores.
Am I really wrong in thinking that it should be cheaper to buy a digital copy of a game than a physical copy?
edit: I must say I love GOG.com though, there I get to pay the same amount as americans, and don't have to be connected to the internet when playing the games I've bought - but their library isn't very big at the moment.