With RSD2 you select which framework you want to use before starting on a project. If you choose Foundation, you will have access to all the components, themes and templates that you find in RFF. The same goes if you choose Bootstrap, all the bits and bobs from RBB are there. RSD2 even has some more goodies that are not built into the other two programmes, don't know if they will in future.
If you buy the Responsive Design Pack, I think it will include RSD2. If not, you will at least be able to get the upgrade for free. If you have built a site in RFF or RBB earlier, you can open them in the new RSD. Projects built in an older RSD version can also be opened in the new one.
When CC brings out a new programme, or a new version, there is always a very good campain price to begin with. Smaller updates to the progs will be free, but big ones most likely will have to be paid for.
I hope you have got some answers to your question with this.