That's a gross oversimplification. Why doesn't Steam follow the same thought on the prices of their games? If people are buying them at full price, why put them on sale?
The answer is: because many times MORE people buy the game that's on sale - these are people who wouldn't buy it at full price. It's a proven, solid fact. Tycho talked about it with their Precipice of Darkness game. The deeper the discount, the more people bought the game, and since it doesn't have a physical media cost, it's all profit. $1 * 1000 > $50 * 15
If they had the DLC pack for ME2 for $20, I'll bet 90% of the XBOX players who bought the PC version on sale would just pony up the $20 instead of hassling with getting their purchases transferred. People who've already played the game and think: "you know, I might play it again some day" would pick it up "just in case".
If Bioware/Origin would bundle up those DLC, and put them on sale, they'd see their sales go through the roof, and I'd bet they could pull in more profit from a 1 week DLC sale (especially if they market it through Steam) than they made on ME2 DLC all of 2013.