Memory cards in general have been dropping in price quite drastically. As far as MSRP goes, it's manufacturer's SUGGESTED retail price. Stores can usually sell for much less, especially in the photography world.
As for Sandisk Extreme vs other cards:
I had a Transcend card die on me at the worst possible moment, basically AFTER taking 100s of photos at an event, and right in the middle of a mass. I was able to recover some of the photos, but a lot of them were corrupt and there was nothing that could be done, even with Transcend's recovery program (comes with their card reader). And the time I wasted switching cards was really bad. Thankfully I was only volunteering, and I had given proper disclaimers before, but if you intend of charging money for your services, invest in a very good card.
After a lot of research, I bought a couple of Sandisk extreme class 10 SDHC cards. As Rorschach mentioned they're not the only mfg of class 10 cards, but they are insanely durable. I read an article where one card spent over a month on the bottom of the ocean floor before being found, and the photos were still all there. I bought a couple of them almost 2 years ago, and they have worked flawlessly.
In my mind, the Sandisk Extreme is the army tank of cards. Sure, there are prettier, cheaper, and possibly even faster cards out there, but if you're going on the frontline of photography, that extra protection is worth the extra cost.
If you don't care so much about the safety of your images, then no worries. A 5$ memory card will work fine; most of the time.
Cameras: Canon EOS 6D, Canon EOS Rebel T3i, Canon EOS Rebel T2i, Canon S90
Lenses: Tamron: SP 24-70mm F/2.8 Di VC USD, SP 70-300mm F/4-5.6 Di VC USD, Rokinon: 8mm Fisheye cine, Canon: EF-S 18-55mm f/3.5-5.6 IS Lens, 75-300mm f/4.0-5.6 III, and EF 50mm f/1.8 II
Retired camera: Fujifilm Finepix s700