The low fare is still there but since 'Q' class is sold out for Aug 29, you're getting quoted a 'H' fare. If you leave on Aug 30, the price will go down to $706.70 including taxes (SWU-eligible Q fare). That's higher than the $581 because Fri is considered as weekend and it's the summer time. If Aug 30 is ok with you, I would advise you book it ASAP.