It's always tough to say with stuff like that. I've seen it be the cable, I've seen it be the power supply, and I've seen it be the logic board. However, if he's already had a tech look at it, then it's more likely the cable like they say.
It's been about 6 months since I've had to replace one of those cables, so my memory is a little foggy, but I do remember hating doing them (which is why the tech charges $400 to replace them). I think it's just annoying because the cables run under other parts, so you end up taking out a decent amount of parts just to get the cables to run to the proper places inside the machine.
It looks like someone published Apple's take apart guide for the 24"....it's pretty similar if you want to take a look: [document|1072]