Expansion packs are on sale for 75% off the regular price, for cash in the Lotro Market web store until Thursday, 12/29:
http://store.turbine.com/store/turbine/en_US/list/categoryID.58733200/parentCategoryID.58516200
Helm's Deep for $10 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 about $10 alone.
The Quad Pack for $10 is a great buy that gives you the same thing as the four expansion packs: Moria, Riders of Rohan (East Rohan), Mirkwood, and Isengard. If you were going to get Moria and Rohan, this gives you Mirkwood and Isengard as well for the same price.
If you don't want the Quad Pack:
Moria for $5 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. Either by itself or as part of the Quad Pack, it's a great value.
Riders of Rohan for $5 includes East Rohan quests, the Road to Erebor instance cluster, and more warsteed options.
For non-VIPS, Mirkwood for $2.50 includes Mirkwood quests plus the popular scaled instances in the Dol Guldur instance cluster. Isengard for $5 includes Dunland, Isengard and Gap of Rohan quests and the non-scaled Isengard instances. VIPs do not need to purchase these as they are included with a subscription.
These are the best prices you'll find for the expansion packs, and they have been offered only a couple times a year for a limited time. The TP prices for the expansions in the in-game store are never discounted this much.