Is it one time cost ?

It's one time cost. You can use your purchased version of Booking Calendar as long as you need on your website.

Also, you can request free updates your version of Booking Calendar (if such updates ready) during 6 months after initial purchase of plugin.

If you will need to extend period of support and getting updates to next 6 months, it will cost 50% from the original cost of your Booking Calendar version (original cost - its cost of Booking Calendar version without other discounts).