Are mockups free of charge?

You can ask payment for mockups, telling the customer you will discount them if they will ask you to continue with the project (e.g. you'll make the pages following the mockups provided).

You can decide (tacitly) to discount less then 100% and take a little amount for the mockups if you invest too much time on them for example.

Never work for free. You could have a different flat fee rate for project discovery or design exploration. In fact, this is a good "hook" package for marketing to sell people on your services. It's also an interview for you too. Get to know the client, process, payment terms etc. You may find out that you don't want to work with them... especially if they try to get free design services from freelancers.

In general, I would agree with the accepted answer, as you should normally ensure that you get paid for your time - regardless of what you do.

However, sometimes a mockup can be essential in landing a customer and could therefore be seen as an investment. Off course, the risk is that the customer takes your free work and goes elsewhere.

Perhaps one could produce 'partial' mockups which demonstrate your capabilities and proposed way forward, but don't reveal how you would go about the entire solution.

The only universal rule in freelancing is always staying focused on turning a lead into a paying customer. If it seems likely that no more work will be forthcoming, all work must be paid for. If more work seems likely, you're in the more difficult grey zone and must evaluate whether the potential client dangling lots of potential future work in front of you is being honest.

As long as a client owes you money - or when you have done agreed upon free work - always ensure that they don't hold all the cards. You must either insist on payment or only make partial deliveries.