Aha, found the relevant patch note:

* Spell Mana Costs: These costs have been reduced for almost all lower level spell ranks. In general, if a spell decreased in cost with a higher level rank in patch 3.2.0, that spell now has the decreased cost at all ranks. In addition, spells learned before level 20 with reduced cast times and/or durations have even further reduced mana costs, proportionate to their reduction in cast time or damage.

The way I read this is that in my example above, HoW would be 12% base mana across the board, and things like fireball for mages (which has a shorter cast time on ranks 1-4) would have even cheaper of a mana cost; i.e. 19% for ranks 5-16 (instead of 23% for most of those ranks) and maybe as cheap as 9-10% for rank one, etc.

Also, when you get past 70, it's easier to get a larger mana pool by way of gear, enchants, and gemming, making OOM'ing harder to do than it was at, say, level 20.

Aren't all spell costs percentage based, so your amount of mana pool doesn't matter, right?

It's based off your character's base mana, AKA mana pool before gear is considered. That way it's consistent across characters and spells 'feel' cheaper as you upgrade gear.

Seems kind of silly if a spell would cost more as your mana pool increased, eh? :)