The expansion is an expansion n_n~

The Standalones are:

Prophecies - One you have.
Factions - Oriental atmosphere.
Nightfall - North-African atmosphere.

The expansion costs less because it isn't as big as the others (well, if we exclude the dungeon content...), and that it really only adds High-end advanced anythings, for people who own any/multiple standalones.

Once you are level 20 you can take a quest from Location: x, and be on yahr way~~


PS: Might not want to read to far into those articles if you can be bothered...