Well actually I haven't ran a vehicle on the same plug wires for that long, but I've owned a couple vehicles (at least) that hadn't had the plug wires changed for about that long. The last one was my '90 Astro, and I never noticed a difference when I changed the wires, even though the old ones did look like shit, and a couple of them came apart as I was removing them.
Darkling, I'm with lee on this one. Be careful about what you get changed on your car. Get a second opinion, preferably, if the first guy says it's going to cost that much to fix it. I've heard lots of stories of mechanics screwing people over because the customers had no clue what the problem actually was.
mdot: Any updates?