A lot of these kind of skills arent always applicable or comparable to a salary position.

Many do odd contract jobs that are extremely high value; i.e. come in a fix this super big bug or add this super important feature on a COBOL system at an extremely high day rate because its hard to find people with appropriate skills.

FWIW when I worked in a Finance company with a lot of Cobol + JCL + DB2 devs (including in management so I could see more info) their salaries were on average similar to Full Stack but possibly lower, especially as we put more AWS emphasis which those people started getting more premium salaries. Some banks I hear give cobol premium but it seems to be more specifically very specific mainframe systems experience + cobol.

But what do those Full Stack engineers make? Salaries are hugely variable across the industry. There are “senior” engineers making 60 k$/yr, and new grads starting at 200 k$/yr

I would say $100k-$175k would be the average range depending on seniority, including bonuses and such.

Generally lower, like other devs in the kind of giant organization that has mainframes.