As Wilba says, try to get the cheapest hotel you can find. Not sure why you want a day room, as luggage storage facilities are readily available at Barcelona Sants train station. Are you staying in a hotel the night before the cruise - if so perhaps you could cheekily ask if you can store your luggage there whilst you have lunch?
Hotels are very expensive in Barcelona. The last couple of cruises we took a stay in 'Hostals' as an alternative. The good ones are in the minority but with careful research you can find the good from the ugly. On our last cruise we pre stayed a couple of nights & then booked a day room for our late flight home. It cost 90 euro between 6 of us & was well worth it. Heres a link you can copy & paste to a very good one only a hundred yards of 'La Ramblas' .....http://www.hostalgrau.com/php/index.php.