If the warranty is cheap that means that it's a solid reliable car most likely. The complexity of modern cars and the likely repairs are all factored into the cost of the warranty.
No, it doesn't - there is no logic like that in the cost of a BMW warranty, IMHO. Something like a 320d with it's catastrophic timing chain issue is a big warranty risk, yet it'll cost the same to buy a warranty for as an N52 engined 325i with no satnav, probably the lowest warranty risk in the E90 range.
It is priced based on the series of car.
Which is why a warranty on a 335i M Sport DCT convertible, one of the more complex cars in the range, is much cheaper than any 5 Series model, even a base spec example with no spec. Take a 520d for example - mechnically fundamentally identical to a 320d, yet the warranty is significantly more expensive. This continues up the range, a 730d is massively more to warranty than a 530d...
I guess nobody but BMW knows how its priced but I would imagine its market based pricing, ie, they charge what they perceive the market for each car to consider reasonable, and a solid manufacturer backed warranty scheme is important for used car desireability and hence residual values, so it probably makes commercial sense for some of the packages to make a loss and be subsidised by other packages.