If a zone is 50% off, consider getting it. 75% off (happens rarely) means getting it, even if it isn't one you'd need or want. At these low prices, just doing the deeds in the pack once gets nearly all your tp back.
1. 595 tp Evendim: One of the best zones and gets you from 30-40 easily. Must-have.
2. 795 tp Angmar: group content, some good instances
2. 595 tp Forochel: solo play
Get either one to lvl but get the other one when you have spare tp or when one's on sale.
(optional) 695 tp Eregion. If you find it on sale or you want to hold off entering Moria, get this one.
3. 1495 tp: Moria: Must-have. There's almost no way around it.
Get the quest-pack if you don't want the classes/character slot. Revamp coming up in U7.
4. 995 tp Mirkwood
4. 695 tp Enedwaith
imo: Mirkwood is slightly nicer to quest in, though Enedwaith isn't bad either. Just get whichever one's on sale first.
5. 3250 tp Isengard
I recommend trying to get the expansion just so you don't need to buy the raise skirmish soldier cap and instance pack separately. However, with the RoR expansion this autumn, I might see this becoming skipable.
Buy it if you're at cap together with the Great River Region.
For those going to spend anyway.
-Do the free zones
-Go through Evendim: by now, you should know if you like the game enough to spend
-Go VIP for a month, log into every character to get trait slots, bag,...
-Buy Mithril Edition: http://mithril.lotro.com/
-or the Isengard Edition: http://isengard.lotro.com/
(One day, I'm gonna turn this into a fancy comprehensible picture.)