The difference in cost between a battery replacement and an engine rebuild is partly market dynamics (our good old friends supply and demand). As more used electric cars enter the market, presumably the supply of replacement battery options increases and with it prices should theoretically drop.

Similarly, there's a possible inflection point where ICE engine rebuild demand drops low enough that some of the very specialized ICE engine parts start to jump in price.

> an electric car with 1/2 the range it had new

This seems a particularly pessimistic assumption given current information. (It's generally fair for the small batteries in a laptop or phone, but cars seem to be trending much better than "half".) The used market seems to be seeing 70% as the "low" figure and actively temperature managed batteries (such as GM's, recent Tesla's, very recent Nissan's) as much as 90%+ range entering the used market. Admittedly, sample sizes are still relatively low because overall electric vehicles seem to have longer than ICE average "first owner" lives (which makes comparing the EV and ICE used markets right now a bit more apples and oranges than people realize).