House of Thule can be purchased at Steam for $19.99. The following things are assumed, it would be best to ask Steam and/or SOE to confirm them:
a.) Limited time only. Most likely this offer will end after the holidays.
b.) For new accounts only. The page claims it will not work for current accounts. I would suggest checking with SOE if you have a current account and wish to upgrade it to HoT.
c.) You must provide a valid credit card to register and play.
d.) You must sign up and install Steam.
Also note that many people are reporting problems downloading and installing EQ through Steam. It is currently suggested that you begin the downloading through Steam, once the download reaches 1-2%, you can right click the expansion in your Steam games list, then "View game properties", one of the tabs has the key which you can enter via the "accounts" page on Sony station.
At that point, you can download EQ through the Sony Station Launcher and then enter the key you need.
Underfoot is also on sale for $9.99 from Steam if you prefer to get that instead. If you get HoT, it includes Underfoot and all other expansions.
This thread on the SOE forums explains more.
UPDATE: Direct2Drive also has EQ at half price, plus a extra 10% discount code "Grinch".