My basic rule of thumb is:
Will the client make money from it, or save time from it?
Then charge for it.
But l’d also say you have to balance pricing as a freelancer. If it’s something that takes you little extra time, but adds value for your clients, consider throwing it in.
But as a bonus!
Make sure they know what it would normally cost.
I see a lot of freelancers going the extra mile, And no-one even knows how big a deal it is.
It’s likely this “little thing” actually took you years to learn.
There is one situation in which I always give away free knowledge…
If you want to offer a high ticket service and charge more than your competition you have to get used to giving away the kind of value for free that others are charging for…
During your sales calls you should be giving out free advice and quick wins to demonstrate your value.
As a beginner everyone told me “don’t give away your knowledge for free”.
But reading a receipe doesn’t make you Gordon Ramsey.
The more you tell people “how”, the more they want you to do it for them.