The dark bars, which come in pairs, are the actual AF sensors used to analyze the light from the splitter lenses.
Each pair of dark bars maps to a single non-cross AF point.
For cross-type AF points, there should be two pairs of bars (that is, 4 bars).
Note that long bars are actually pairs of small bars, not single long bars.
The center AF points in both the 5D and 7D are double-precision points, so the thicker bars are presumably the ones for the center AF point.
Also, the center AF point in the 7D is cross type in X-formation, so the 45-degree bars seen on the 7D AF chip are obviously also for the center AF point.
Other than that, it's a guessing game which bars map to which AF points.
The general principle is that a) the longer the bars and b) the further apart they are from each other, the more accurate the AF point is.
I don't have an idea about how expensive AF chips are but the same principle applies as with normal sensors â€“ the larger the size, the more expensive the chip.
And for lots of high-precision AF points, a large chip is needed.