The nature of steam and its sales makes that sort of calculation sort of fussy. It's not like you can get rid of Steam games (yet), so it's entirely possible that I game I bought today won't be played for another year or two. Yes, economics states that money NOW is more valuable than money in the future, but I think that's splitting hairs (even more than I'm splitting them now).
If I buy a game on day 1 for $50 and don't play it until it's already dropped to $20, it's safe to say that $30 was spent unnecessarily. Steam sales, however, will generally put a game at the cheapest price it will ever be, so waiting a good deal of time to play them won't really change the cost of doing so.
But if the point is that steam sales will just nickle and dime you out of the same cash you would have spent on a full-price purchase, I can't disagree with that.