The way I understand it, the BCM is one more computer in the car. Some cars have quite a few computers- I was reading in Car and Driver a while back about some production GM car had 6 or 7 computers. I am a computer weenie and have worked in the electronics manufacturing area for many years (in the past, not now). I can tell you that the engine computers in most cars are quite inexpensive to manufacture, given that they are one simple PCB with a few chips. I bet the cost to GM of the basic engine computer was probably in the range of $15 to $30. Of course, this does not really cover the cost of generating the specific calibration (the program, maps etc.) which is tailored to each specific car / engine combination. I have never taken apart a BCM so I cannot comment on that other than to speculate that its probably even cheaper than the engine computer.
About the Ignition Switch being a Saturn F.U.- turns out the exact same part is used in the Cobalt, Pontiac G5, Chevy HHR and a few other models. The TSB on replacing the ignition switch (not the key part, but a plastic switch/relay like thing on the end of the key lock) says that if replacing the switch did not work, then replace the BCM. That concerns me because that part from a dealer is probably expensive, but replacing the switch worked for me.
i think the point of adding "if replacing the switch doesn't work, replace bcm" is that the two things that control whether or not a signal is sent to the pcm to crank the motor are the ignition switch and the bcm and of course, the wiring. if there is a no-crank/no-start with security tell-tale and potential body codes, and you replace the switch and the problem still exists, then it must be a faulty bcm. however, since it is probably always the switch, even if next year the problem recurs, there should be very few people having to replace bcms,
in other words, if yuou fix the problem once with the switch, then next year the problem comes back, don't automatically assume now you need a new bcm. try a(nother) new switch and if the problem is not solved, THEN you need a bcm.
Q: What's the difference between a bolt and a screw?
A: First, you screw. Then, you bolt.