i just bought a 64GB sandisk extreme UHS-3 for $80 at best buy. right now is the time of the year to buy memory cards. they will be going for super cheap because of black friday. if you dont load up you will have to wait till next year. when it comes to prices at places like best buy, cards go for dirt cheap for black friday.
i suggest you grab one especially for 4K work you will want a 64GB which is the largest card the gopro will recognize.
Minolta XD11, 28/2.8, 50/1.4
Canon C100mkII, Canon 5DmkIII, 7D, 100/2.8 macro, 70-200/4L IS, 1.4X mkIII