Season tickets. A lot of people buy them without attending every game. Barça averages 75-80k for the Camp Nou but sells 90k STs.
Yeah did a bit of research after that, didn't realise they have so many on the waiting list.
It seems they have downgraded the stadium project, including a reduced capacity of 61k, so that they may complete the ground. No idea if the Peter Lim bid, if successful, would mean going back to the original 75k capacity. Might as well settle for more than a 6k increase over Old Mestalla if they have the numbers. The original stadium plan looks great too.
http://stadiumdb.com/news/2013/11/valencia_new_plan_to_deliver_nou_mestalla_smaller_and_cheaper
Read that Atletico is scheduled to move into a new 70k capacity stadium in 2015. Another major advantage to add to the footballing front.