XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (327 page)

is incorrect (at any rate, it doesn't do what you probably intended), because the context item doesn't change within this expression, which means that the relative path expression
@price
is not evaluated relative to each item in turn. The way you should write this is:

It's easy to forget that a path expression such as
//item
requires a context node. This expression selects nodes starting from the document node at the root of the tree containing the context node. So when the context item changes, if it selects a node in a different document, then the result of
//item
changes too. This also means (which can come as a surprise) that when the context item isn't a node, an expression such as
//item
gives you an error. For example, this means you can't write: