A horde is cheaper to start with, as the "hard hitters" tend not to come in plastics. You can start with a horde, and slowly build up your elites later.
You'll probably want the Mordor sourcebook if you don't already have it. Some of the armies in there can ally with each other, so you can be pretty flexible about how you structure your army.
I'd get a box of orcs, a box of morannons, and a box of warg riders. Then you have a choice to make about bows: orc bows suck, so if you want bowmen you'll need to use orc trackers, which come in blister packs of 3 each. If you don't care about bowmen, then you'll need to be able to engage the enemy more quickly, so an orc drummer is essential. Personally I like drummers, it gives elves a lot less shooting time
That's probably in the $100 or higher range already. Now you need a few elites and some good heroes. A few Mordor Uruk-hai sprinkled in adds some higher fight and punch. An orc Shaman will keep your troops in line. An orc command blister gets you a banner and captain. I also really like the look of the morannon orc captains. In general, named captains are a better deal than unnamed captains, so you might want Sharku to lead your wargs (allied from Isengard). If you have part of your army from Cirith Ungol or Minas Morgul, you can take Gorbag or Shagrat which come as a pair. Finally, a wraith is kind of necessary if your opponent fields any mega-heroes. If you can find a generic ringwraith model, foot and mounted, that will give you some flexibility as to which ringwraith you want to use in each battle, unless you've already picked a favorite.
Anyway, that should get you something reasonable for around $200. I didn't mention trolls, because the model is very expensive given your budget. Hope that helps.
