iFixIt checked out what Apple's $49 Thunderbolt cable is made off and discovered the cable uses two Gennum GN2033 chips in the connector, one on each side, plus a couple of smaller chips.
IFixIt tore the white cable apart and found a pair of Gennum GN2033 chips hiding beneath the sheathing, with one on each connector of the cable. In total there were also 10 other smaller tiny chips and an assortment of transistors, etc.
Gennum's webpage brags that its chip takes normal cables and offers "sophisticated signal boosting and detection functions required to transfer high-speed data without errors."
Of course nobody seems to know how much these chips cost so it's hard to say exactly how much profit Apple is pulling in off the cables.