When the Normandie was taken down, the SI was just over 50k (approx). After that incident, John added more SI to bring it up to over 80k. So, the current "take down cost" is around 3k peds ammo at its current SI, assuming the repair crew is similar to what it was the other day. The repair crew was not particularly numerous at the time; at the end there were only 2 repairers on my section, IIRC, though it was not the section that failed. The fire was mostly concentrated on one section. A heavy repair crew is the only thing that will keep a MS alive under a sustained attack, as everyone can keep on clicking repair for hours and no attacker is going to sit out there for hours wasting ammo. This is also why it's important to have the RK-20 maxed -- one RK-20 does 3x the repair as one RK-5, and population on the ship is limited. A high SI gives several advantages; it gives crew time to react to an attack and run to sections that are undermanned or maneuver, launch quads, get to safe zone, log out, call for backup, etc. It'll be interesting to see if there's a max SI for these ships.
Oztwo, I agree, before selling the Motherships, Mindark should have informed us how much it would cost to upgrade the SI. I think the bids would have been much lower had people known they were not getting a battle-ready ship out of the box.