Wired on the success of Settlers...

[Whaleyland]

Buy the expansion from BoulderGames.com. Mayfair sets their in-store and on-line prices but Boulder claims that is bullocks so has the cheapest price online. It may not be cheaper after shipping, but some other sites like TimeWellSpent.org usually price matches if they have cheaper shipping (I have done this at least three times but not in the last year). Just my advice. I am a student so rarely can afford anything I buy, hence why I go price shopping frequently.

[meepleater]

Just done more research and changed my priorities a few times, most recent list in order of priority for expansions:

Already Have:
Base Game
Traders and Barbarians

Want to get (in order of priority):
Seafarers (I still think I'll get this first!) Got T&B anyway. Now it really is first
Traders and Barbarians
Base game 5-6
Sefarers 5-6
T&B 5-6

Might not get:
Cities and Knights
C&K 5-6