Unless there is something really screwed, the cable should be more than adequate for home internet use. I have access to a DTX through my company and have taken it home many times for personal testing. It takes an incredible amount of abuse to slow down your average 5e or 6 cable. For a cable to be rated 5e or 6 it must be capable of a minimum throughput.
Even the cheapest cable will be good if it's not physically damaged and it's terminated correctly.
Exactly, the runs aren't long enough in most homes to matter, even if you lose a little, it isn't worth the huge markup.
Not to mention to get the most out of it, you need more than just the cables. A $35 Belkin router isn't going to do the same as a $200 router. For one or two systems, in a low use environment you wouldn't see much difference, but load it up with a lot of large data transfers and some torrents and watch what happens. As with everything in computing you need a balance, and it goes the opposite way as well, investing in 10gigabit wires and cards won't help if your hard drives can barely flood a 1gigabit connection.
Contrary to how it looks, it's actually not that easy to flood a gigabit connection with just two average computers. I've done testing with several routers, switches, computers, nics and drives, and most have trouble doing much better than 80% of the theoretical throughput. Reading a conventional hard drive can exceed gigabit, but writing is much closer, SSD's can flood it pretty easy even for write speeds, but few have SSD drives at both ends. You can have the greatest network around, but if your system can't push more than 50% of the bandwidth it's wasted, unless of course you have a lot of systems pushing 50%, but for most homes, there is rarely more than one or two people taking advantage of the bandwidth, and rarely at the same time.
More than that, you don't need a full gigabit to stream hd movies, wireless G can do it if everything is in good shape and no one else is using it, so while you don't want wireless G, 100 or N 150 will suffice in most cases. There is plenty of wireless options between 150 and 900 as well. So really unless you have a file server, there's really not even a whole lot of need for gigabit in the home as only large file shares really take advantage of it.
I'm not saying don't do gigabit, wired is far more reliable, and there's no reason to go slower, just that you shouldn't spend extra trying to achieve another 5% throughput. Especially when you don't know where your bottleneck is to begin with. Use good routers/switches, and some generic cat5e and call it a day, unless you find a SEVERE bottleneck, which occasionally happens, but not often. Usually you find that the N.A.S. is your bottleneck*, not the wires.
*FAR more common than people realize, home N.A.S. boxes suck.