Older expansion packs are on sale for 85% off the regular price, for cash in the Lotro Market web store until Thursday, 11/30:
http://store.turbine.com/store/turbine/en_US/list/categoryID.58733200/parentCategoryID.58516200
These are the best prices we have seen for these expansion packs (Previous best price was 75% off) and they are available for a limited time. If you think you will ever want them, now is the time to buy. Mordor, the latest expansion, is not on sale in the Lotro Market. However, it is now available for 2495 LP in the in-game store.
Helm's Deep for $6 includes West Rohan quests and Helm's Deep Epic Battles (aka Big Battles aka BBs), which can be played by characters of different levels since characters are all scaled to 100. It also includes 1,000 Turbine Points, which is worth more than $6 alone.
The Quad Pack is not on sale. However, buying the four expansion packs (Moria, Riders of Rohan (East Rohan), Mirkwood, and Isengard) separately is only $10.50.
Moria for $3 includes Moria quests, skirmishes, and some great non-scaled instances. It also includes the ability to play Warden and Rune-Keeper, plus 2 more character slots. It's a great value.
Riders of Rohan for $3 includes East Rohan quests, the Road to Erebor instance cluster, and more warsteed options.
Mirkwood for $1.50 includes Mirkwood quests plus the popular scaled instances in the Dol Guldur instance cluster. Isengard for $3 includes Dunland, Isengard and Gap of Rohan quests and the non-scaled Isengard instances.
These are the best prices you'll find for the expansion packs, and it has been a year since they were on sale.