Mana Cost of Animating Undead

The printed mana cost of 4 per HD of undead is correct.

This means the Necromancer's ability to create powerful (ie high HD) undead is very limited for his level. A couple of thoughts on this:

• Mearls apparently intended to top undead HD out at 8 all along: see the "HD by size" entry in the table on page 242. The link between size and HD (and other stats) is now gone, but the limit remains intact.
• Yes, 6-8 HD at levels 17-20 is, in terms of combat ability, useless. But that 6-8 HD creature can have many other uses. For instance, a Necromancer with a large flying zombie minion has permanent flight capabilities for himself: a very powerful tactical ability in the Iron Heroes context. And he could have several such beasts to allow his whole group to fly if need be.
• Limiting the max HD of each undead increases the number of undead servitors a necromancer will have (since he can't just have 1 or 2 high HD ones), and explains why even high-level necromancers have standard skeleton minions (which is a genre convention).
• Undead minions are permanent until destroyed. Giving them a higher max HD means that each use of Necromancy becomes like a permanent mid-level conjuration, which is pretty darn strong.