Because A's fans are awesome, Friday June 28th, 2013 is "Fill the Coliseum Day." (See #fillthecoliseum on twitter.) It appears the day is being organized by the Pie Guy. Here's his blog about it: #fillthecoliseum. The A's will be playing the Cardinals on their only visit here in 2013 (until the World Series).

There was a groupon available for 1/2 off tickets and over 4200 tickets were sold at $14. 

No more groupon tickets available but you can still buy tickets here and there are still cheap seats available  (and expensive ones) which is great, because that's how the A's are! (And the Coliseum is very affordable!