[Anaraxes]

I wouldn't think of that as adding an extra step, so much as modifying the rules by which the attack operates. It doesn't do infinite damage; it does a variable amount of damage based on a damage roll, the scale of which takes the target's HP into account.

The touchstone for an attack that just kills someone would be an Affliction of the Heart Attack condition. In terms of damage, a disintegration effect is usually taken to be the amount of damage to take the target to -10x HT -- or, for a humanoid target, 110 points, which works out to a 33d Innate Attack (or a 0d+110 attack, if you prefer). Disintegration is usually treated as the Corrosion damage type.

I'd price a Limitation of a d100 roll for damage, treated as a percentage, times the base value of the attack as a -50% Limitation -- in other words, half the price of the attack, because on average it will do half of its rated damage. I'd use the flat value of 110 points in this case to avoid two rolls. Might even tweak the cost down to make it 100 points, just so the d100 becomes a damage roll with no need to multiply. A little less lethal, of course.

Basing the damage total on the target's HP rather than a pre-determined amount calls for an Enhancement, IMO -- if I allowed it at all. That effect lets the attacker potentially one-shot any tough opponent (dragons, Ogre tanks, gods), which means the campaign is going to be full of BBEGs that conveniently are immune to the attack for various handwavy plot reasons. Occasionally that effect limits the damage, if you're smashing small things. But that's less risky in the first place. So, probably +20% at least, maybe more if I expect the campaign to feature lots of big tough opponents rather than swarms of little ones (say, the classic "Against the Giants" series of D&D modules...)

Sense-Based (B109) means that an attack affects the defender via one of his own senses. It doesn't mean that the attacker has to make some sort of Perception roll to hit. Sense-Based is a "penetration modifier". Its primary mechanical effect is causing the attack to ignore DR. (An attack that did infinite damage wouldn't need to ignore DR; no finite amount of DR would reduce its damage at all. So why pay for the Enhancement?)

There is a Limitation for an effect that requires the attacker to be able to sense his target ("Sense-Based, Reversed", Powers p105. This Limitation only applies to abilities that normally ignore DR anyway. (Otherwise, it wouldn't be a Limitation.) You could apply both Sense-Based and Sense-Based, Reversed to the same ability, meaning that the attacker would have to see (etc.) the target and the target also see the attacker. You might want both on an ability where both characters have to lock their eyes, as with the Harry Dresden soulgaze.

Most attacks require the attacker to be able to sense his target with a targeting sense; see, for example, the penalties for darkness or Blind Fighting. Generally that shows up as a targeting penalty, not a separate roll.