When we first released our storyboarding app Boords, we put in a feature which allowed users to make new versions of a given storyboard. Initially this was only included in the paid plan. If a user on a free plan tried to make a new version, they saw a popup prompting them to upgrade to a paid plan.
No-one clicked upgrade on that popup. Literally no-one.
Versioning, while a useful feature, was not important enough to our customers for them to pay for it in isolation. Clearly we needed to try a different tack.
We made Versioning free, and without changing anything else, saw conversion rates go up across the app. We'd made the app more useful to everybody, so, perhaps obviously, everybody used it more.
Versioning alone wasn't important enough to charge for, but combined with everything else the app did it markedly increased the overall value of what we were offering.
How to decide?
Much of today's business wisdom extols the importance of giving away as much as possible;
By giving away value and helping others as much as you can, they’ll respect you; it will build your Reputation, but it will also increase the probability that they will be interested enough when you do present your Call-To-Action. Josh Kaufman – The Personal MBA
Help people solve their problem before asking for any money, and when the time comes the value of what you're offering will be obvious to them. But how much is "as much as you can"?
The key to deciding whether to charge for something is to understand exactly where your product provides value. Is it at the core of the reason people hire your product? If you're asking for money for it, it needs to be.