The other day over on Don't Forget Your Boots, the subject of the extreme cost (and broad utility) of the Power enchantment came up.
There is a good idea on that blog, but I thought of another solution.
One way to deal with this is a limited version of the Power spell.
To do this more-or-less by the rules as written, you can tweak and use the Attune enchantment, which functions as a category limiter. Amusingly, you have to pay for the Attune spell, too.
Power and Attune
The Attune enchantment can be used to limit the number of spells covered by an item's Power or Speed enchantments.
One Spell: 20% cost
2-3 Spells: 40% cost
4 or more spells: 100% cost
You can also limit it by college:
One Specific College: 60% cost (same as a one-college powerstone)
Two or more colleges: 100% cost
Importantly, the floor on enchantment costs here is 100 energy. Like the Deflect enchantment, this one falls onto the $20/point of energy cost side of the cost divide. So even Power 1 (One Spell) is still 100 energy and thus $2000. Equally, Attune should cost $2000, too.
For example, a Ring of Dark Vision with Power 5 costs:
Dark Vision: 400 energy
Attune: 100 energy
Power 5 (One Spell): 1600 energy
Total: 2100 energy x $20 = $42,000 plus the cost of the underlying item.
$42K isn't cheap, but you're getting Dark Vision, always on, no cost to cast or maintain, no need to activate it.
However, this does make certain items very cheap.
Iron Arm: 600 energy
Attune: 100 energy
Power 1 (One Spell): 100 energy
Total: 800 energy x $20 = $16,000 + the cost of the underlying item.
$16K is peanuts for a free automatic sword parry every turn in DF. Currently, it's 1100 energy and thus $22K, which is still cheap for its value.
Optionally: This covers cost to cast and maintain; if you only do one of those, its only 75% cost. Figure the cost for the highest level of the Power needed for both casting and maintenance. Figure the difference between any additional levels of casting-only or maintaining-only Power, and then apply the discount to the difference. Blocking spells cannot use this limitation, and must pay full cost for Power in order to take advantage of it.
Power 5 (One Spell) is 8000 x 20% = 1600 energy. Only to cast = 800 energy.
In combination, it gets trickier:
Power 3 (One Spell) is 2000 x 20% = 400 energy. If it also has Power 5 (One Spell, Casting Only), the additional cost is 8000 (Power 5) - 2000 (Power 3) = 6000 x 20% x 75% = 800 energy. Total cost for this item is 1200 energy for 5 Power for casting one spell, 3 power for maintain that same one spell. For the Dark Vision item above, it'll be 1800 energy or $36K plus the underlying item.
Notes: I haven't tried this one yet at all. I'd love to have people try to find abusive uses of it so I can settle on a fair cost. The "casting only" cost might be really abusive, and makes Missile spell items really cheap (and since many spells don't really need to be maintained, not terribly limiting - take a look at, say, Watchdog) On the other hand, it does make spells with an equal cost to cast and maintain a much more expensive item to make free to cast and maintain.
Don't forget, though, that items can have their own power reserves - check Dungeon Fantasy 8, p. 48.
Again, this isn't a tested rules suggestion. It's purely an idea at this point, and it desperately needs people to hammer on it.