I would make the recipe cost something like Pul + Uber standard. It would just appropriately increase the value of the highest runes and make the good runeword slightly more accessible, particularly to SSF players. But I don't think it would break the game. Grinding down should always be considered trading for worse, regardless of what the market feels about zod vs ber or Jah vs ber. There's no break chance for grinding up, which in theory gives you something better. I don't think grinding down should have a flat break chance for nothing though. So Zod = Cham + (Jah, Ber, Sur, Lol, Ohm, or nothing- odds depending). Maybe down even down break guarantees 1 of 1st level below and 1 of 2nd-6th below like your chart, with odds for the 2nd rune to poof entirely and be a 1:1 break. I don't think a perfect grind down is appropriate, but a down break into 1 also feels bad. It'd put the high runes not used for as many rune words at a more proper value for their grind up cost. I'd be on board for a loss of a quarter every break. %'s can be adjusted to make the system more or less fair depending on community sentiment. Downgrading a rune will only ever yield 1 rune, has a 50% chance of poofing the rune entirely, and a 10% chance of hitting the rune you want or 40% chance of hitting a rune close to the one you want.

For D2 I could see there being a combination.

There are likely others, or some combination of these. This way your rune doens't just go poof, and you get something either usable or may be able to upgrade soon. So if you broke a zod it could turn into a cham, Jah, Ber, Sur, or Lo. Penalty 3: % Chance to make any rune from the nearest 5 tiers. Penalty 2: % Change of failing (Recipe just consumes the rune) this may be tuned to make it viable, give players a chance at taking a high rune that is worthless into one that can be used for a key piece. Penalty 1: 1 Rune downgrades to 1 Rune - So Zod downgrade to 1 Cham (this would be a minimum, as splitting 1 Zod to 2 Cham, and so on would instantly break the game)

Diablo 2 rune downgrade