PREFIX  dc: <http://purl.org/dc/elements/1.1/>
PREFIX  x: <http://example.org/ns#>
SELECT  ?title ?price
WHERE
    { ?book dc:title ?title . 
      OPTIONAL
        { ?book x:price ?price } . 
      FILTER ( ( ! bound(?price) ) || ( ?price < 15 ) ) .
    }