how much does it cost to use the personal hotspot software?

Using the personal hotspot software on the iDevices doesn't cost anything, individual carriers may charge for the connection, however. Some in the US are charging $20/month for the service.

This suppport service pagegives some pricing in the US.

It may or may not cost anything EXTRA. Just remember that even if it doesn't, all the data you use will count to your data plan.