I posted in another thread about this; I made a Greater Flaming/Frost/etc. set of enchantments for my campaign. +3 cost. +2d8 damage, they give off continuous light, don't need an action to activate (they're always on, which can be good or bad), and each has a secondary effect with DC 15 to avoid.
For example, the Flaming one sets the target on fire (see DMG for rules, but basically it's 1d6 damage per round until you make a Reflex DC 15 save). Most of the secondary effects are based on the Orb spells from T&B. Two use Reflex, two use Fort, one uses Will; DC 15 isn't very high, so it's not going to dominate the high-level game, but it'll still be useful enough to justify the higher market price.(Tweak stats as needed for balance, but that's the basic idea)
It's not that allowing a Sword of Really Hot Fire (i.e., triple Flaming) would be unbalancing, but there's just more stylish ways to do this than to create a stacking headache. If you let Flaming stack, then people ask for Flaming Burst, and if you let that stack, why not Keen or Wounding?
KaeYoss: (1d6+1d6) may be worse than 2d6 against a single type of resistance, but on the flip side, you're more likely to have damage he DOESN'T resist. For example, a Fiendish-template creature has Fire and Cold resistance. I'd rather have (1d6 Fire + 1d6 Lightning) than (2d6 Fire) against him, but both of those are better than (1d6 Fire + 1d6 Cold).