How much would even greater Chasuble of Fell Power items cost?

[Nebuul]

Chasuble of Fell power at CL 6 is +1d6 damage and 8k gold.
CL9 is +2d6 and 18k gold.

How much would it cost to do 3d6-to-5d6 versions?

CL 12 = 3d6
CL 15 = 4d6
CL 18 = 5d6

I was thinking the cost to build is (1+n)^2*2. So 1d6 = (1+1)^2*2 = 8k
2d6: 3*3*2 = 18k
3d6: 4*4*2 = 32k
4d6: 5*5*2 = 50k
5d6: 6*6*2 = 72k

Do those numbers sound reasonable (not too strong, not too weak)?

[Troacctid]

It's the same cost progression used for magic weapons.

[Nebuul]

Ah, thanks!

Edit: So is the Lesser Chasuble considered +2?

[Troacctid]

That's how I'd rule it, yeah.

[KillianHawkeye]

Edit: So is the Lesser Chasuble considered +2?

Not explicitly, and with only two official examples it's impossible to know for sure if they'd continue to follow that pattern. But considering how well the formula fits the two official chasubles, it's safe to assume that's what they were going for and price higher ones as you've done already.