Is it costly to develop a native application for the desktop?

To do it yourself, no. It costs your time and whatever you might have made doing something else with that time.

To pay someone else to do it, yes. The cost generally scales with the complexity of the app.

However.. I suspect the 'native' in the question means you're asking if native development costs more than hybrid development using Electron (or something similar). In that case the answer is that it's more expensive to write natively, but not by as much as you'd think. A lot of the work on an app is actually very reusable between platforms (art assets, sounds, business logic, algorithms, project management, etc). The code itself isn't very reusable, but that's usually only around 30% of the total budget, so writing a native app for two platforms ends up being about 30% more than one platform because that's the bit you end up doing twice.