Ring of Mystery magic item help

[Arcanus]

This ring is from FFE's free web enhancement of Rings of Power.

The ring has the following powers:
The Ring of Mystery has one power only—the ability
to go unnoticed (even by its owner, when it isn’t being
worn). A Spot check at DC 35 is needed to notice the
ring itself when it is lying out in the open. If it is hidden
amidst other treasure, the DC becomes 45. This ability to
go unnoticed also affects the wearer, though at a reduced
level. A Spot check at DC 35 is needed to notice the
wearer in a crowd, or at DC 25 to see him standing
alone. Anyone specifically looking for the ring or its
wearer reduces these DCs by 5 points.

Now my questions are: What is the cost to make this ring
including any xps and spells needed to create.

How would this ring effect the wearer if he had a damn good
hide skill that he was trying to use to begin with?
Would it increase the spot check needed to find me?
Would I roll my hide skill and add that number to the spot DC from the ring?

 

[Quidam]

Start near, then adjust

I'd start with a ring of invisibility and head up from there, and fast. Look at the DC's to spot invisible creatures and compare them with yours. Since you're not fading from sight, it's mind affecting- you're making them not notice you. So there are certain types that will be immune to that. You should also give at a radius of effect, since it's mind affecting rather than illusion.

 

[Arcanus]

Any more helpful insight on this?

I kinda got that idea at first but was still unsure on where to go from there.