Why is there an assumption that these items would be available in the corner shop?
Attribute-setting items should (especially in point-buy) be a decision by the GM, not something someone can just make.
But if someone wants one, and it doesn't seem unbalanced, why not let them come across one? From then on, as long as they wear that item, treat their stat as the items value.
As for the item's cost? Who cares. It's not the sort of thing that's for sale. (Although setting a stat to 18 from 6 =/= a +12 bonus that could take someone from 18 to 30. It's worth a lot less)
