No the salaries aren't high. They are typically lower than other software engineer salaries. There are a large number of contractors from Indian consulting companies with "experience in cobol" to make run of the mill cobol cheap enough.

The very high salaries you hear about sometimes are always for VERY specific mainframes that are extremely old with a lot of quirks, and are usually being paid to consultants.

I suppose it varies by country?

In my country, no, COBOL jobs aren't well paid. They are below average.