I'm guessing you, and many others for that matter, think that since they have their own distro, they must be coding themselves almost everything they use. This is simply not true. Simplified version is they just select what software they want to use and install it off the official Ubuntu repositories.
No, I don't think this at all, but I would expect the level of effort to be similar to something like CentOS. Probably more so, since if I understand correctly, one of their goals was to not be tied too tightly to their upstream distro, so they'll be faced with having to replace libblahblah.so.4 and all of its dependencies when they want to update packages foo, bar,and blee that upstream decided can't change for stability purposes.
I will say that your point about the work involved with maintaining a golden Windows image is a good one, though given that DLL hell is mostly a thing of the past (I won't comment on the shitty way that MS dealt with that, but it is more or less fixed) it's probably a lot less work than the above. Still, it is a point I hadn't considered.
Their claimed cost savings is something like $20M, so that pays for a LOT of overhead. But does it pay for enough?