Direct 2 Drive are selling the Sentinel's Fate expansion for £4.95 today ($4.95 in the US). Even with the horrible exchange rate it's a really good deal. Usual price is £29.95.
The deal includes approx £4 of Station Cash which makes it cost just 95p for the expansion.
Sentinel's Fate is necessary to unlock the deepest AA abilities and to access the 80-90 areas and to level past 80.