Following the announcement of JamUp's support for Audiobus on Monday, they are having deep sales today. JamUp Pro XT is only $5! This is a new record for the app, which has previously only ever gone down to $10!
The same deal is available in the free JamUp XT, where you can purchase the Essential Expansion Pack for just $5.
For exsisting JamUp users who have purchased any of their IAPs in the past, you might want to take a look at this forum thread on KVR, with lots of details on how you can upgrade, but not cross-grade.