Sales don't cost Valve money. Sales gain Valve money. They make less money per sale, certainly, but since the only cost per purchase to Valve is bandwidth the vastly higher quantities sold often make up for it.
And even if they don't, the other big component of the sales is that suddenly a hell of a lot more people are playing that game. They may talk about it, they may not; but because you can always look at what your friends are playing that guarantees Valve another form of free advertisement. If seeing your friends play it and investigating is enough to convince you to buy the game immediately, they got a full-price purchase from that initial discounted purchase; if it isn't, there still may be a seed of interest left in you for when the game comes up for sale again.
Steam's success is built on ludicrously cheap sales in a way no physical store can match. I wouldn't consider them at risk of collapsing from that model any time soon, but Steam's monopoly might be bad for the industry if Valve starts getting complacent. Happily, though, I haven't seen signs of it yet.
And even if they don't, the other big component of the sales is that suddenly a hell of a lot more people are playing that game. They may talk about it, they may not; but because you can always look at what your friends are playing that guarantees Valve another form of free advertisement. If seeing your friends play it and investigating is enough to convince you to buy the game immediately, they got a full-price purchase from that initial discounted purchase; if it isn't, there still may be a seed of interest left in you for when the game comes up for sale again.
Steam's success is built on ludicrously cheap sales in a way no physical store can match. I wouldn't consider them at risk of collapsing from that model any time soon, but Steam's monopoly might be bad for the industry if Valve starts getting complacent. Happily, though, I haven't seen signs of it yet.