All valid sound points.
Provided that the 2 extra sites are for development and test only, your live site license will cover them fine too, and will work already now technically.
So no need to purchase an additional license for development and testing.
In case you have another live site used in productive live site, you would then need to either extend your existing license (button "Change options" inside license on your profile here) to a second live site, or to purchase a separate license ("Your current subscription upgrade possibilities" section below your current license).
We might reword our license slightly at next revision to expressly put that case in it.
Thanks for asking and suggesting sound solutions. In my first reply I didn't notice that it was your dev=development and test subsites, but understood they were productive. Sorry for fast read while releasing CBSubs 1.0.2.