Necromancy on a BudgetEdit
OK, normally it wants you to pay 25 GP per hit die of undead, and that’s more than a little stiff. I’m not even going to pretend that you’d be willing to spend 50 GP for a human Zombie – those guys aren’t better than 1st level Experts, and those guys only cost 1 SP a day.
A funny little detail is the fact that you don't even know how much onyx to use: HD is an abstraction your character is not supposed to know about. The spell Trap the Soul does say that its possible to research someone's HD, so take that for what you will, but it doesn't really say how long it takes to research Bob the Fire Giant Mook's HD, and frankly you don't care; if you use the incorrect amount, the spell fails and the slot is wasted, but you don't lose anything else. This means your onyx is not used up if you fail in the casting, so if you are not in a big hurry, you can simply cast the spell over and over (each time adding 25 gp of onyx) until the spell works and you burn up the minimum amount of onyx (in a combat or other situation where time or slots might need to be conserved, you can just grab a handful of onyx gems and hope you have enough for the casting).
The really funny thing fact: I’m not even sure how you’re supposed to get these onyx gems into the eye sockets or mouths of the creatures you are animating. Onyx isn’t all that valuable, and 50 GP is a whole pound of gold: really a decently high hit die creature should require the placement of an Onyx bigger than its actual head inside its eye socket/mouth. That may require uses of Shrink Item if your DM is actually using material components as written. If you don’t want to get involved in that argument, consider raising your army of the dead for cheap or free, use scrolls or staves with Animate Dead, or do one of the following:
Play through The Sunless CitadelEdit
The Azun-Gund only cost 3 grand. They aren’t even hard to make. They make 2 Zombies that’ll follow you around, and can make Zombies of up to 10 hit dice. Further, that’s per party member, you can jolly well pass that whistle around. A party of fourth level adventurers can have 8 zombies running around with it at all times.
Be a Pale MasterEdit
Pale Masters suffer a little bit from the fact that they kind of blow. They lose actual caster levels and don’t get any abilities at the time. But they quickly get the ability cast animate dead for free, and eventually get to punch people in the face so hard that they join your army of the dead free of charge (and without limit). With enough patience, you can have an undead army of literally unlimited size free of charge.
Hand People Unholy ArrowsEdit
Unholy Arrows are very modestly priced. They cost about 361 GP each. And any Good aligned character takes an actual negative level if they hold it in their hand. A character can wield however many arrows happen to be in their quiver, so they can potentially get a lot of negative levels. Any character with negative levels equal to their hit dice dies and rises as a Wight, and the arrows are not consumed in the process. Repeat with Holy, Lawful, or Chaotic arrows as desired.
Maybe you’re supposed to Try to cast Animating fireballs, but that’s like a 6th level spell. What you actually do with it is make Animating Acid Splashes that you use on enemies that have been dropped and haven’t bled to death by the end of combat. It’s like a death knell only it adds to your zombie army. It’s available as your 5th level bonus feat as a Wizard (precisely when it becomes available for use).
Have a Spell-like AbilityEdit
Spell-likes default to a single standard action and bypass all components (including XP and GP). So if you have Animate as a Spell-like ability you can raise all the dead you want. Really. You can get this by a number of feats, special class features, or the old standby of using Planar Binding/Ally on Fiends that happen to have Animate Dead as a Spell-like in the first place.