It also occurs to me that portion sizes have perhaps increased. If you a Google image search for "portion" "sizes" "over" "time," you get all kinds of charts making this claim. I wasn't around in the 1950s, but it does seem that, at a minimum, soda sizes have increased. I vaguely remember that it was common to get a 10 ounce bottle of soda 30 or 40 years ago. I haven't seen a 10 ounce bottle in years; it seems that 16 ounces is the standard single serving bottle size and 20 ounces is pretty common too.

Here's an article which seems to agree:

http://abcnews.go.com/WNT/story?id=129685

Hamburgers have expanded by 23 percent; A plate of Mexican food is 27 percent bigger; Soft drinks have increased in size by 52 percent; Snacks, whether they be potato chips, pretzels or crackers, are 60 percent larger.

So if you look at things in terms of dollars per calorie, the decline in the price of prepared foods may very well be even more dramatic than it seems on the surface.