Value is always a tough one to determine. I think your statement "just in the eye of the beholder" is as close to any real answer you will get. It depends on what someone is willing to pay you for the chip. For insurance reasons I would just insure it for the full book value.

On a personal note, I like chips for their artistic and historic value, so if it were a chip that I really liked then the yellowing would only add "character" to the chip for me and not detract from it's value.

So, I would say much like beauty, value is in the eye of the beholder.

Take Care,
Tom Hanson