Here is my two cents...
If I were you, unless you need them right away, I would not buy the two individual expansions. I would buy the Big Box 2. It comes with the base set (will give you your 2nd) and four large (Inns & Cathedrals, Traders & Builders, Princess & Dragon, Abbey & Mayor) and four small expansions (The River 2, The Count of Carcassonne, Cult Places, King). The King expansion is only 5 playable tiles. If you buy the King & Scout expansion, it includes an expansion for Hunters & Gatherers whish is a spin-off and not playable with the regular game. I've heard some people say it's quite fun though itself, so you still might be interested. The Cult Places is 5 tiles and is the same as the Cult, Siege & Creativity, but that one comes with 6 cult tiles.
The Big Box 2 can be found online for less than $60. It's quite a bargain. You can then supliment it with the expansions you are missing. The one I'd most highly recommend is the Games Quarterly 11 expansion which contains 12 mostly unique tiles and can be found most easily on ebay. Buying the BB2 leaves you with only the Tower, the Catapult, Cult, Siege & Creativity, Wheel of Fortune, and the brand new
Tunnel from Spielbox, and the
River 1 which is now available.
Unless you can find everything you want real cheap on ebay,
Fairplay Games has great prices and a really good deal, free shipping on orders over $125 which you can get very close to with all the expansions at one time.
HERE is a comprehensive list of all the official expansions and information on them. Hope this helps.
Don't forget to check out our fan-made expansions
HERE.