It is a one time payment and you are purchasing a perpetual license with undefined term of upgrade rights. I recently purchased it myself so I dunno how often that an upgrade fee is required, but it looks like the last time was version 8.x (where JRiver is now on version 19), here is where it says permanent (perpetual). If there is a major release that requires upgrade purchase, there is still the option to continue using the latest version before an upgrade cost with your perm code... as long as you want. http://wiki.jriver.com/index.php/Restoring_a_License
Also, during the first 14 days you are allowed to use your install key to activate the app from several installs... covering your home PC(s) as well as work PC via one license purchase (each generated key is machine specific, and you can generate new keys for your PCs 10 times a year). That is pretty awesome IMO. Reference: http://www.jriver.com/purchase.html
Q: Can I install on more than one PC?
A: During the two-week period after you have purchased or restored, you can use the Install Key you received to activate the program on your other PC’s. You may use the program on your work PC and on your home PC's with a single license.