The Sheets command importXML
reads only the HTML source of the page without executing any JavaScript on it. As you can see yourself by using "view source" in the browser, the source indeed has "-" in that span:
<span class="price_big_right">
<span id="ps-lowest-1">-</span>
</span>
The actual numbers are loaded by some JavaScript file which then inserts them in that span. Neither importXML nor other Sheets functions can retrieve dynamically inserted data.
Sometimes, after inspecting the JS files, one can uncover the URL of source of data and try to import that; but this is a tedious reverse engineering exercise for each particular site.
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