Let me tell you the way of things. As Goa said, Rebellion is $30 if you have registered your Sins original and you buy from this site. If you lost the copy and the manual with the code on the back, well then I guess you will just have to shell out another $10.
Rebellion is a stand-alone expansion, which means that it does not need any of the other expansions or the original game to run. Actually, it includes all of the content from the original game and the two micro-expansions, Diplomacy and Entrenchment. This means that Rebellion is like the ultimate edition of Sins, containing every bit of content from the original, the two micro-expansions and then Rebellion.
Also, I should mention that Rebellion has a lot of new and improved stuff in it and I would urge you to buy this as it improves upon Trinity in almost every way possible apart from removing the limits of the Iron Engine (graphics, AI, multiplayer, balance, titans, more capitals, corvettes, dwarf planets, more music, new UI, new background for the main menu, other cool stuff I can't think of right now)
There is also a GREAT collector's edition being sold on this site for Rebellion. Check it out:
Enjoy this awesome game and welcome back to the Sins community, franown.