If you want a cheap flight from Barcelona to Berlin, you've got zero airlines to choose from.
You can use the filters on your search to see more flights and find cheaper rates – Berlin has more than one airport. So if you don't need to fly into Berlin-tegel / Otto Lilienthal Airport, it's worth checking your other options.
During this peak season, hotel and holiday prices often increase too, so it's best to book early. Or you could keep your fingers crossed and hold out for a last-minute deal.