I bought MJ8 when it was the latest version at full price. It was possible to use MC9 and 9.1 for free with an earlier MJ8 license. I bought an upgrade to MC10 and also to MC11.
Currently JRiver offers an upgrade from MJ8 to MC11 Beta for $15. I think it is a good bargain. I don't think there is any reason to upgrade to MC10 since MC11 will be ready quite soon.
MJ8 lists now $19.98 and the full version of MC11 is $35 (this may change later to $45 as seen here:
Media Center 11 Prices). Actually, if you bought MJ8 recently you get in a way full refund if you buy the upgrade now.
One thing that is often underestimated is how good MC10 and MC11 are in handling of audio. One of the reasons for that is JRiver's own comparison table:
http://www.jrmediacenter.com/comparison.html. A reader easily gets an impression that the audio part of MJ8 and MC10 is the same.
MJ8 users are often saying that they don't need the extra capabilities, but they don't realize that MC10 and MC11 are far superior also with audio files and in basic database handling. Perhaps JRiver could bring that better up.