If you want to stick with name brand cards here's a deal on Lexar 64GB SDXC Professional Class 10 600x UHS-I Memory Cards. Regular price $69.95, save an additional -$27 dollars OFF.
These cards are fast enough to support 4K video from the new GH4 and 1080p at 200Mb/s. For a limited time these 64GB 600x SDXC cards are now just $42.95 (found here).
Lexar 64GB 600x SDXC Media Cards