calculate currency for a custom field

Hello,

I have a multilingual and multicurrency website. I would like to have a simple product available on backorder, which has a price brackets like e.g. 1000 z? – 4000 z? (add to cart button needs to be turn off, because these products needs to be ordered individually). The issue what I have is:

The right side of bracket are not calculated, when it is switched to another currency e.g Euro, and it shows like this: €220,00 – €4.000,00 (what obviously should not be €4000, it was 4000z? and it stood like this, it changed only sign.

Custom field I have added with solution from Stack and following code:
https://stackoverflow.com/a/50503257

Could you help me with calcuting the price correctly?

Best regards,
Alan

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