Something that’s important to keep in mind here is........ depending on how the cam is configured...... the difference between the headers and manifolds could vary greatly.

A cam designed to minimize the losses when used with manifolds isn’t going to gain as much by going to headers as the cam that essentially requires headers is going to lose by running it with ex manifolds.
One might show a 20-30 difference....... the other could be close to 100hp.

The takeaway is....... there is no standard answer as to how much headers are worth.

As with just about everything relating to making more power...... “it depends”.