Also hopefully batteries will get cheaper over time.
Yes.
> How much does that cost including all parts and labor as a percentage of the vehicle's value?
I think you are overestimating the time it take to replace a battery with appropriate equipment.
https://www.youtube.com/watch?v=23qFN9sLjVU
Initially only leased batteries were available, so there is still a lot of people with leased batteries, and batteries get swapped for free when capacity drop below 75%.
The trick is that the battery capacity rarely go below 75%.
Battery replacement is something very rare (which make leasing a bad deal), 10 years laters there is still very few peoples that peoples that made their car batteries replaced.
A 2002 model year Toyota 4Runner in decent running condition still retains significant value today, and can be easily repaired with commonly available parts. In 2042, will we be able to say the same about any EV being sold today?