One problem with many offensive magic items is that the DC is to low to be useful for the price. For example, a Crystal Ball costs 42,000 gp, but a DC 16 Will save negates. If you spend 1/4 of your wealth on this, you are at about 14th level. At that level, even characters with poor Will saves can usually make a DC 16 save.
The standard formula for this (use Heighten spell to increase the spell level in the calculation) provides a slow increase for a lot of money. DC 17 (based on a 5th level spell) costs 9*5/7*4=1.6 times as much, so +1 DC for a 60% increase in price. The next increase (to DC 19) is another 46% increase. At 23%/+1 DC, I suppose it might be considered a bargain. However, 115k gp is one quarter the wealth of a 18th level character and the targets will save has probably also gone up by almost as much.
I am contemplating the following house rule.
Magic Item DC: When creating a magic item, the DC to resist its effects can be increased. Each +1 to the DC adds 5% to the cost of the item.
Applied to the above crystal ball, a crystal ball with a DC of 26 (+10 above normal) would have a cost of 63k gp: +10 DC = +50% cost, 42k + 50% = 63k.
My reasoning for the cost is as follows. Increasing the DC is most valuable for save-negates effects. Each +1 to the DC is an additional +5% chance to fail the save, so the value of the item should only actually increase by 5%.
This could be applied to scrolls, but I think I would want the caster to be able to cast that spell with the given DC, similar to how the caster currently must have a high enough mental stat. For example, if Willy the Wizard has an Int of 15 and Spell Focus(Evocation), he could read a scroll of Slow with a DC of 15 or less, but could read a scroll of Fireball with a DC of 16. (As with any scroll, it only matters if the spell is on his spell list, not in his spell book.)
The logic there is that a DC 16 Fireball is a more complicated spell than a DC 15 Fireball, much like a 4th level spell is a more complicated spell than a 3rd level spell. If you are not "smart enough" (or wise enough or nice enough) to cast the more complicated spell, neither can you manage to cast it from a scroll.
Thoughts?