An Undead Bane weapon costs an extra 6000 GP on a +1 weapon and does an extra 2D6+2 damage (8000 GP).
An extra 10000 GP on a +2 weapon (18000 GP).
An extra 14000 GP on a +3 weapon (32000 GP).
An extra 18000 GP on a +4 weapon (50000 GP).
An extra 22000 GP on a +5 weapon (72000 GP).
Assuming, of course, that the weapon has no other properties.
The proposed Ring would do an extra 1D6 damage at level 1, 2D6 at level 3, ... up to 10D6 at level 19. Plus, it would stack with any weapon.
That's extremely potent. A lot more potent than the Undead Bane weapon. Even the +5 Undead Bane weapon only does an extra 2D6+5 damage (plus a little more for hitting more often). Compared to an extra 10D6 damage, that's pretty light.
So, the 10D6 version of item should cost considerably more than 72000 GP.
I would also put a limit on the maximum Sneak Attack damage possible. That way, there could be different versions of the Ring, all of them at a different cost:
+1D6: 2000 GP
+2D6: 8000 GP
+3D6: 18000 GP
+4D6: 32000 GP
+5D6: 50000 GP
+6D6: 72000 GP
+7D6: 98000 GP
+8D6: 128000 GP
+9D6: 162000 GP
+10D6: 200000 GP
Even thought many Rogues might get a cheaper one, very few would pony up for the more expensive ones. This, I think, is balanced.