Cost of personal hotspot

See Share your internet connection from iPhone - Apple Support (CA)

Note: Personal Hotspot isn’t available with all carriers. Additional fees might apply. The number of devices that can join your Personal Hotspot at one time depends on your carrier and iPhone model. Contact your carrier for more information.

and

When you share a Personal Hotspot from your iPhone, it uses cellular data for the internet connection. To monitor your cellular data network usage, go to Settings > Cellular. See View or change cellular data settings on iPhone.

So, if you can do it with your service provider (at all), it may cost whatever extra they charge, or it may just eat into your data usage.

You would need to check with your cellular provider to determine if there are additional charges for using hotspot tethering.

Charlie-01 wrote:

Is there a fee for using a hotspot?

If you are using someone else's hotspot, then no. Any extra charges or data usage is up to them. It should be like using a public wi-fi.