There are differences in the $7 vs $100. build quality can change considerably, mostly in the connectors though and not the cable. When you spend the extra money you are usually spending it to get better connectors, as has been stated, the cable is pretty much the same.
On the cable, as long as the bandwidth of it can support the frequencies that you are transmitting, there is not an issue.
I would watch out for the super cheap cables, I have seen many of the connectors fail or the plating on them chips off (if there is some on there, I question some). Best bet would be to look for a name you can trust (Monster, Belkin, Acoustic Research, Tera Labs, etc...) and check out Ebay, you can usually get a good one for a good price. Most online vendors will also have them at pretty good prices.
enjoy!