The client software has no cost. However the download page requires a service contract on an eligible ASA to download the current version. Otherwise you are limited to the version included on your ASA.
On the ASA itself the default license is 2 AnyConnect Premium client licenses. From there you can change to AnyConnect Essentials (which allows basic VPN for as many clients as the hardware supports) or additional AnyConnect Premium licenses (Premium is licensed in tiers - 10 user, 25, 50 etc.). Both of those "server-side" options have a cost.
