If we introduce the concept of “scaling with level” to any ability, it would be an argument for all abilities.
I mean, the game doesn't currently have any defensive ability whose usage causes damage to the person who used it. Mettle is adding that. Doesn't that set a precedent for all defensive abilities needing to have a BP cost to use? (Honestly, if Mettle needs this, than its probably the case that spell-parry needs this as well.)
Honestly, I think its a bad precedent entirely, and that isn't how we should be scaling mettle.
Attempting to take half a step back, and establish some perpsective:
Mettle is providing a smart defense against a bunch of mostly magic effects that would take a fighter out of combat. The 2.0 changes include removing cloak/bane magic items, which can change high level play, some of which is desirable to preserve. We want fighters to be able to close and be in combat. At the same time, we don't want magic to be useless, so we can't have fighters able to soak a lot of magic in a short time.
You can purchase one mettle per 20 martial skills. It costs 3-6 XP per. This makes it one of the two cheapest defensive abilities, along with evade.
Its likely closest equivalent is Dodge, which applies to more effects and attacks, scales per 30 points in stealth skills, and costs 5-8 XP per.
The problem I think is mettle's current scaling is to harsh for non-straight fighters who aren't high level; and for low level fighters. I think it _should_ be usable by a fighter, scout, or spellsword of all levels - we have no other level restricted skill. In addition, because of the addition of hardy, a 40th level fighter who aggressively bought hardy _could_ use mettle 14* times in a row (* - actually 13, since you couldn't buy that much hardy and qualify for 14 mettles), which I think is definitely not intended. Interestingly, if a scholar had a way to get access to that many mettles, they could use it 11 times in a row.
Without Hardy, to use 1 more mettle a fighter needs 10 levels, a scout 14 levels, a templar 18 levels. A fighter can first use mettle at 6th level, a scout at 9th, a spellsword at 11th.
I'm starting to think the addition of Hardy makes Mettle scale in really unintentional ways.
Interestingly, it also makes mettle really good for spellswords who can heal themselves. You could conceivable close doing the templar-tuck (one hand free behind a shield), suck up a relatively arbitrary number of effects and be healing yourself after each. And I don't think this ability is intended to be better for spellswords than fighters.
Make Mettle & Hardy be one skill. Expending it removes the effect of the Hardy. I think my suggested variant is buyable every 4 levels, provides 10 body instead of 5, at 10-14 XP per purchase (twice current hardy), and using the skill removes the effect of the Hardy until the next logistics period. It'd likely make sense to double the effect of frail/hardy racial abilities in this case.
Make Mettle purchasable like dodge and remove the BP damage. Increase prereqs appropriately. Any skill-store restriction on dodge should apply equally to Mettle. Possibly make it slightly cheaper XP wise than dodge, since it doesn't cover some key effects dodge does. (We already have evade<->parry parallel, I think adding a dodge<->mettle isn't inherently flawed.)
Make Mettle have a secondary effect instead of damage. (This does set a precedent.) I'd suggested using Mettle cause weakness. But you could possibly make it progressive - something like first hit is slow, second is weakness, etc.