Clarification on Expression indexes

Greetings all.

The following page: https://www.postgresql.org/docs/11/indexes-expressional.html

States the following:

I'd like to get an idea on "relatively expensive". For instance, compared to a partial index, which is split on one or more boolean values. As opposed to making a function for this that serves the same identical calculation.

Let's say that over the lifetime of a row, it rarely gets updated more than 2000 times, and the majority of this is right after creation?

Is this a concern?

Regards,

Koen De Groote