Oh I don't think it would be too costly - you could definitely still use the uruk-hai bodies if you just found some replacement heads (Hasselfree sells a sprue of orc heads that would probably work just fine). You might even find a use for those Minas Tirith heads if you wanted to do some fiefdom conversions using different bodies.
Alternatively, you could use Wargames Factory samurai bodies, not sure where you'd get appropriate heads and arms, though (unless once again using uruk heads, and I'm sure you could source arms from somewhere else).
For Starks, I think Fireforge foot sergeants are probably the way to go, although you might want heads from the Perry WotR sets for those rounded helmets.
For Stannis's troops, these might make good helmets:
Spanish Cavalry Desert Rider HeadsThe helmets on Renly's troops remind me of a particular style of hoplite helmet, the name of which I can't recall. The first helmet on the left in
this image is pretty close, though. That's a victrix set, and I know the Immortal Classical Hoplites set (now sold by Warlord) has at least a few of said helmet as well. Certainly not economical unless you want the hoplites anyways, but you could ask around and see if people have any spares. Or, maybe you have a friend who's interested in building a hoplite army, and you can just nab the heads off of him?
For both Baratheon factions, the armor looks pretty generic although you might have to greenstuff some cloaks on. You'd also have to add the stag antlers to Renly's troops' helmets.