Player came across a stack of 2 gray stones in a shop and kicked one.
That one ended up with a different (in his case, lower) price once it
was separate. This behavior only applies to non-glass gems which add
a price variation derived from internal ID (obj->o_id) number. Make
splitting stacks always yield the same price per item in the new stack
as was being charged in the old stack by choosing a similar o_id. Do
it for all splits (that can vary price by ID, so just non-glass gems),
not just ones performed inside shops.
He picked up the lower priced one and dropped it back on the original
higher priced one; the combined stack took on the lower price. That
will no longer happen if they come from splitting a stack, but this
fix doesn't address merging with different prices when they start out
as separate stacks. (Unpaid items won't merge in inventory if prices
are different, but shop-owned items will merge on floor.)
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