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 (?title = "TITLE 2") .
          }
        } .
    }