4 mil yen so about 25,255 usd (as of may 2026) which is still crazy expensive and my bet is most of the price is the magic water stone they need. I went back and reread a few chapters but I couldn't find a price point of magic stones. But based on how they are kept like precious gems (chap 19 pg 12) i'm going to guess they are expensive.
For anyone wondering the 1 gold second hand crapper would have been 1 mil yen/6,315 usd
Thanks for the chapter!