1. I don't know the real reason they went with the MB seat, but as with any multi-million dollar contract, there were certainly more than a couple of reasons. Personally, I don't believe it was money well spent. And if the people involved in "upgrading" to the C-model were involved... well, that's a whole different story. There was a much better cockpit offered and they went with the lowest bidder... which ended up costing the most and being less capable. But I digress...
2. I've never heard the official cost for the seat upgrade, but simple math means it was big. When the first jets were going to get upgraded at DLF, MB looked at the parts of the system that the AF was responsible for (IIRC, it was the rails) and wouldn't touch it until the corrosion situation was improved. I cannot verify this, but DLF folks told me there was at least 1 MB person at DLF doing very little for over a year while the problem was addressed.
3. Had the AF saved the money for the C-model, PMP mod, and ejection seat... and had a comprehensive plan earlier on for acquiring the T-X, the money and horsepower spent on those individual projects would have gone a long way in getting the T-X procured by now. Yes, I'm sure those with PEM/staff experience will state "that's not how it works"... but maybe it should. Piecemealing improvements to the T-38 during that ~10 year window cost a ton of money, and I don't believe it was money well spent.
4. I did not realize that there were successful ejections on the MB seat that would have been fatal on the Northrop seat. Glad to hear that. But how many were there? Divide the cost of the program by that number and ask yourself if that was worth the expense. I'm not a believer in "if one life was saved, it was worth it". It's a silly justification, and we cannot afford it.
5. You state the MB seat upgrade saved a person/people... but I believe there is one person that died in an MB seat that would have survived had the Northrop seat been the seat he was flying on.
6. As a side note, the T-38 I'm flying now has the original seat. A few of the owners (of private T-38's and F-5's) have asked about going to an MB seat. I believe the huge cost to do so erased that idea, and I'm pretty sure all of them will stick with the old seat... even the ones that could afford it. Going to a cold seat isn't an option like it is on an L-39 and other jet warbirds: I'm happy to fly an L-39 with cold seats (and I do)... but not a T-38 or F-5.