If you research eBay for pricing, it is important to only look at Koyamaichi chisels, and no other makes. Such as this ...
https://www.ebay.com/itm/325952221319
That is an 18mm oire nomi and it is $108 USD.
Here is a set of 5 Koiyamaichi - from Lee Valley - selling for $375 ...
https://www.ebay.com/itm/17630050585...Bk9SR7rnu9HfYw
There are many - many! - Japanese chisels on offer that are cheap - but they are nameless.
I have Kiyohisa chisels and, if I wanted to sell them (which I will not), I would be guided by other equivalent Kiyohisa chisels on sale.
Regards from Perth
Derek