How else should they be evaluated? Genuine question.While I understand what you're going after here, I'm not sure that a direct build equivalency can (or necessarily should) be drawn with many of the Paragon Path abilities, and everyone will value them differently (sometimes dramatically so). One person's 10 build is another person's "that's junk, why would I ever use that?"
It's safe to say that most players will be giving up 50-build worth of Combat-related skills/abilities. to get these Paths.
If we are meant to spend that 50 build for these, and we are giving up "so much", how else are we supposed to evaluate what we're giving up (50 build, presumably spent in combat abilities)? It's easier to figure out build difference/equivalents of each Path and their relative power levels.
It's difficult not to do this since every PC-available skill/ability up to this point has a Build Cost; this method will be best when the Path one takes gives PC-available abilities, giving it a "build equivalent".
For example, if I use a skill that costs be 2 BP to buy, but allows me to use a Skill/Ability that would cost me 50 Build to acquire, that's important to know and gives an idea of that Paragon Path's relative power. It's important to the would-be user of the skill, and it's important to plot so they can know the relative power of for scaling purposes.
If I ever end up buying into one of these Paths, this is the method I will likely use unless a better one is presented; and I think it's a method that becomes even more important the higher level you are so you can - again - get a better idea of what you're getting versus what you'd be giving up. I think this will be true regardless of whichever path a player is trying to evaluate.
If path gives me a "free" eviscerate at level 45, and I have already bought all of the Eviscerates I can, I can save build by buying the Paragon Path instead of waiting for another 15 build to make sure I have the 60 build pre-req and the 5 build cost of buying the skill; this is important for high levels because build is acquired so very...very...very...very...very...very...very...slowly...