Advice on setting cost for function

How do I determine what value to set as the COST of a function? I understand that it’s a relative, abstract, per-row sort of thing, but that’s not quite enough to go on to do it well.

If I have a function that looks up a single value from an index, should that be lower than 100? 20, say?

If I have a function that calls multiple other functions, each of which does a reasonable amount of work, should I set the caller to a higher COST, or will Postgres use the costs of the functions it calls?

I can imagine that a SQL function which is just inlined might ignore the COST estimate. Is that so? Are there other such considerations?

In general, a section in the CREATE FUNCTION documentation of two or three paragraphs with a few examples and general guidelines would be very helpful. I would be happy to write it if someone will explain it to me.