It's all a big scam!
The signal is digital, so as long as it can get through (i.e., it's a conductor and not a few pieces of string in the cable), it doesn't matter.
I recently bought a �3 HDMI cable from amazon. I check in Currys, and their version (same length) was nearly �50?!!!
I feel sorry for the fools that would buy a cable for that price.
Get the cheap one, it'll work perfectly. There is some v1.3 HDCP stuff that you might want to make sure your cable supports, but all new ones should do now anyway.