The burrowers can be irritating, but are very situational: Their poor RAT
makes it hard for them to hit unless it's a heavy or you've debuffed their target. With Grim's feat, they're solid gold. Otherwise, I'll typically leave them at home.
The Fell Caller is wonderful, although rather dependent on infantry swarms. Mind you, you're playing Trollbloods: If you're not fielding swarms of Tough infantry, there's something wrong with you.
Near the top of my list would be the UA
for the Krielstone. It's certainly not necessary, but the extra bonuses are gravy to the larger fury bank.
You'll probably also want a heavy of some sort. You can't really go wrong with any of the Troll warbeasts, but my personal favorites are the Mauler and Earthborn. Mulg is also disgustingly good, but is a wee bit out of your suggested price range if you want anything else. More lights can work too, and I'd personally recommend a Pyre or Slag Troll (the latter, if your meta is WM
heavy) for their animi.
Finally, infantry: Your two main choices are the Fennblades and the Kriel Warriors. Monetarily, the Fennblades are the cheaper of the two, as the Kriels fall under the "Fifteen Medium Based Metal Models" umbrella of pricing. The Kriels are a hellish tar pit and can hit rather well between the Fell Caller, their chant, and a Kriel Stone (plus a Caber or three). The Fennblades, on the other hand, are an extremely effective offensive unit that punishes your opponent for thinning their ranks.