should 2+ Vulnerabilities give diminishing returns?

[Plane]

B46's table is used, so "Vulnerable to Lead" and "Vulnerable to Steel" are occasional threats, while "Vulnerable to Metal" would be a Common threat and "Vulnerable to Physical Attacks from any Material Substance" would be a Very Common threat.

If we're looking at the x4 values, occasional is -40, so a pair of occasional x4 would be worth -80.

This ends up not only being more than "all metals" but tied with "all physical attacks". So it would generally always be better to take a pair of Occasionals instead of a Common/VC one.

The best I can think of is something like Alternative Abilities where you take the most expensive vulnerability and any which are cheaper are only worth 1/5 as much.

If you did that, then "Silver or Lead" would only be -48, rightfully fewer than "All Metals" at -60.

[Say, it isn't that bad!]

I recall that there is an inherent, built-in factor in the costs to compensate for the fact that, once people know your secret weakness, they begin preparing for said secret weakness. That is, I remember this coming up, and that explanation being given.

[Varyon]

IIRC, there are a few cases of templates with multiple Vulnerabilities that only get the discount for the worst. That said, using 1/5th cost for further Vulnerabilities in the same (or lesser) category is probably roughly fair.

[Plane]

Originally Posted by Say, it isn't that bad!

I recall that there is an inherent, built-in factor in the costs to compensate for the fact that, once people know your secret weakness, they begin preparing for said secret weakness. That is, I remember this coming up, and that explanation being given.

You can prepare for "use metal against my foe" more easily than "steel or lead" though, since you have extra options. The problem is "steel or lead" giving more points when counted separately.

[Say, it isn't that bad!]

Originally Posted by Plane

You can prepare for "use metal against my foe" more easily than "steel or lead" though, since you have extra options. The problem is "steel or lead" giving more points when counted separately.

...Sleep. I need it.

[Anaraxes]

Originally Posted by Plane

You can prepare for "use metal against my foe" more easily than "steel or lead" though, since you have extra options.

I'll just pop out to the sporting goods store and get some osmium bullets and a rubidium-technetium alloy hunting knife, just in case. That's easier than steel or lead.

At some point, you're just decomposing one category into relative availability of its components. "Any metal" is in some mathematical sense more available than any two metals, but the customs, technology, and economics of the setting are what really influence how common something is.

I think one simple principle for evaluating a proposed build is that subcomponents can never add up to more than the base category. I'd raise an eyebrow at any multiple Vulnerability just from the point of view of "how does this make sense in the character concept?", and would happily suggest just bumping up the category to cover multiple related banes.

The "prepping for the enemy weakness" is a worthwhile consideration, too. What are the odds of a fragment of another planet winding up on Earth? Not zero; we have Martian meteorites, and Oumuamua shows that even planets from another solar system aren't mathematically impossible. But how much kryptonite actually shows up in Superman stories? The stuff is practically common, in multiple varieties. GURPS Disad values have a strong "do they affect the game" narrative quality ("a disad that doesn't affect the character isn't worth any points"), rather than just a pure simulationist model of a random sampling of the world environment. "Common" means "likely to come up in a story" more than it does "likely to be found in a random kitchen drawer".