I've been goofing around with the leather values to try and come up with a more reasonable progression of values. Here's what I have.
Hmm, let's see how these compare to their metal equivalents. From the manual:
Leather = Leather
Tough Leather = Copper
Studded Leather = Scaleplate = Shellplate = Chitinplate = Bronze
Lamellar Leather = Rigid Chitin¹ = Rigid Scale² = Iron
Rigid Shellplate = Iron++
Nether leather³ = Dragon scale = Steel
¹ Rigid Chitin is iron grade, but very light, and so should be worth more
² Rigid Scale is iron grade, but fireproof, and so should be worth a tiny bit more.
³ Nether leather has a fixed temp, making it slightly more valuable than dragon scale.
Copper has a material value of 2, and when made into bars would be worth 2×5 = 10. However I believe it requires only a *single* piece of leather to make armour (which would require 3 bars), giving tough leather an equivalent value of 30. However it also takes a single piece to make a leather helm (would would only require 1 bar). So I like your in-between value of 20.
If we're using metal equivalent material value × 10 to figure out the leather equivalent, then this would put studded leather, scaleplate, shellplate and chitinplate at 40 (based on bronze = 4)
Lamellar, rigid chitin, and rigid scaleplate would be 80 (based on iron = 8 ). Rigid chitin and scaleplate may be worth more, since they're light and fireproof respectively. I value lightness over fireproof, so perhaps 90 for rigid scale, 100 for rigid chitin.
Rigid Shellplate would be worth 120 if we model it on the value of rusty steel (12). However given that rusty steel is apparently 80% as good as regular steel, it may be undervalued in the raws. Giving Rigid Scaleplate a value of 160 would put it mid-way between iron and steel in value. (The manual notes it's heavy, but I don't know if it's heavier than its metal counterparts)
Nether leather and dragon scale are apparently steel grade, light, and cool. They should be worth *at least* 240 (based upon steel), but probably more since they're both light *and* fixed-temp, so I'd suggest 360.
Of course, all of these values are great in terms of figuring out how much it's worth compared to metal bars, but there's also the question of whether we'll see crazily-expensive items once crafted. Socks have a x6 multiplier, so normally a 'metal' sock would be worth just slightly more than a bar of the same material. However if leather gets multiplied the same way, then a dragonscale sock would be worth 1800☼, whereas a steel sock only 144. 'Armor' has a multiplier of 21, so a 'steel armor' would be 504, but a dragonscale armor 6300. That's high, but since dragonscale is about 1/5th the weight of steel, it doesn't feel unreasonable. This does put masterwork dragonscale armour into artefact-level value ranges, but a masterwork dragonscale armour sure *sounds* like an artefact to me!
One could argue the values I've suggested could be halved, but I wouldn't be inclined to make them much lower than that.
Just as an aside, I would *love* to rename 'Rigid Shellplate' to 'Hardened Shellplate', so all the 'rigid' items have the same qualities. One could even put forward an argument to rename 'lamellar leather' to 'rigid leather'.
TL;DR: The new leathers should be worth a lot more.