Interesting...... Two things to add here.....
1) I don't think nikonor's fix is quote going about this the right way.
2) I don't think this is only a session issue, although @TarGG58, would be good if you could state what you did that worked for you. Did you just logout / login? Was it anything to do with nikonor's post? It might help others if you could clarify.
I looked into this briefly due to a client project, although the solution I wanted was not about 'fixing' this, so haven't followed up yet (due to time). Neither the client's site, nor my test site had dynamic price updating (version 2.02, Joomla 1.7) on quantity change. A variety of Jquery configs (google api on / off, vm jquery on/off) in VM backend didn't affect this.
What I'm going to do is test this with the latest release, different server, latest Joomla. It may simply be my / clients setup. If not sorted, I will see if client's work can be converted to a fix here for this (assuming I get their permission).