MrVamire is right about petco king's betta. They are not real giants, just old betta and they are at their maximum size for a round tail plakat. I don't know why petco call those fishes king, they would just make a lot of confusions for some of you and also to make more sells.
I know a store owner in Sacramento, he sell the same kind and same size for $4.99 each and i believe petco should sell their "kings betta" around that price as well.
I don't think you would see a real giant either at petco or petsmart, they can not even afford to get good quality betta with good colors, i dont think you will get giant hm or giant hmplakat.