This is actually not too common requirement so the market is small, therefore amount of sold units small as well. That means the price per unit will be high no matter who makes it.
However, I believe there is a cheap solution! Many routerboards come with USB. I know for sure that it supports at least some (but likely most) USB-to-RS232 adapters. You know - those typical gray/cyan cables, often marked as "cisco console cable" and sold for few $ on ebay.
It came into my mind just now, but I believe if you plug an USB hub into routerboard, you should be able to connect multiple of those adapters, therefore getting many (possibly up to 127?) RS232 ports per routerboard.
EDIT: yep, it works. I plugged both my USB-Serial adapters via a USB hub. Both connected, both working at the same time:
winbox_KDnfs6BH6r.png
With this approach, I am confident you can create almost unlimited amount of consoles for just few $ per console...
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