I think Beholder's eye's crystal disk would be better suited as a Focus.
Since it sounds more expensive and difficult to get a hold on just for a bonus to appraise. But a focus would be great, use it multiple times and maybe use it like a magnefying glass.
For kicks, a bonus to search as well would be cool. Go all Sherlock Holmes with your fancy new detective gear.
Interesting idea, also just so you know the disk doesn't need to be large it might only be the size of a ring. That would cut the cost.
Tiny gems are still rather hard to come by, reasonably. If your DM plays into more of the role aspects.
I was thinking of the saying "it takes money to make money" except because it doesn't have a listed price it technically doesn't cost anything.