At Adorama .. Lexar Platinum II 64Gb SD (SDXC) cards for $11.95 with free shipping. These are pretty fast (Class 10, 200x). I bought 2.
Lexar Platinum II 200X SD Card
Lexar Platinum II 200X SD Card
Well .. these are sold out now anyway.
I use 400x compact flash in my 5d and 7ds (both have SD card as a 2nd card). I have used 100x SD cards as well .. only time it's an issue is for sports when I'm firing off long strings, since the 7dII will do 10 fps.
Usually, if I'm paranoid about losing something, I'll set the 5d or 7d on dual write and write to the CF and SD simultaneously .. not sure if the 6d has that. It also jumps to the 2nd card once the first fills up if not in dual write mode.
You take things much farther than I ever had .. I typically will reuse a SD card once I've released the photos.
You also shoot raw, which chews up more than the JPEG I shoot.The 6d is a single SD card. It doesn't have dual media, like the 5d. With the numbers of shots we're doing, not sure I'd want to do that anyways. We usually use 2 cards per camera body.
I shoot L-JPEG .. only because I need to process so many shots .. and I'm not getting paid for it. A track meet 3 weeks ago I shot 2400 images and need to process it down to 200-300. Picasa lets me go through lots of images quickly, but doesn't handle raw well. SO JPEG until I spend time with Lightroom to learn it properly.
I've been lucky then .. never had any issue with a card and I've shot close to 200K images (125K on my original 7d alone).
I'd like to learn it to apply some standardized color correction, white balance, etc. to a group of shots. Also it has lens correction and far better sharpness and highlighting/fill control. I played with mine a bit, but I couldn't get the process down well enough. I was using LR3 .. I picked up LR 5 .. now it seems they want you to move to the Adobe CC platform for $100+ a year.