Like a replacement engine for an ICE car, the replacement cost of a complete battery pack is probably not relevant to figuring TCO, except as an upper worst case margin.
For example, an engine replacement (which is remanufactured) for my 911 is about 15K + core. But if something goes wrong with the engine, chances are that the subassembly will be replaced and not the entire engine.
So it will be for replacement battery packs for EVs such as the LEAF. If there is an entire battery pack offered up as a replacement, it will most certainly not be new (or virgin), but rather remanufactured, with all of its components brought up to current standards. And over the life of the vehicle, if the battery pack needs maintenance or repair, it will be repaired at the sub-assembly level. So a pack will have only the faulty cells or components replaced, and only after some threshold would the entire pack be replaced with another one. The old pack core, would then be remanufactured, and used on another vehicle. The recycling and remanufacturing of the core pack keeps the actual cost of repair or replacement as economical as possible. I would imagine that Phil's $5K quote includes a core exchange, and that it is not a new pack but a remanufactured one.
From what I can gather about replacement Pack costs, they are quite in line with other major assembly replacement costs for conventional automobiles. And given that the pack is quite modular, they will probably be less.
2018 LEAF SL
Delivery April 10 2018
2014 LEAF SV
Delivery May 23 2014
50,000+ miles - all 12 bars - Same range as new - No warranty issues ever!