I would buy Pink Floyd (the best prog rock band ever), Yes , Jethro Tull, Supertramp & Genesis (as long as it is from Phil Collins era) dlc.
I wouldn't buy any "prog" metal dlc cos I don't like it, and can't listen to it without getting a headache.
Seriously The Beattles and Queen labeled as prog rock, of The Beattles I would believe you mentioned Sgt. Pepper album but Queen? And I do like Queen