Hub asssembly and wheel bearings?

I have a 1998 Olds Cutlass. I've experienced vibration in the steering wheel, especially at high speeds. A friend who does a lot of his own car repairs said it sounded like my bearings were going bad. I've done a little work(brakes, etc.), so thought I'd take a look myself. The first thing I discovered was that I couldn't find a parts store that could sell just the bearings. They all said it was a "wheel hub assembly". Okay, so after looking at evrything, I decided to have a local shop look at it. I took it in one of their guys took it for a drive, put it on the rack, played with the wheels, and said he couldn't find anything wrong. I told him it certainly didn't feel right. They said they'd have another guy look at it.

Okay, so I get a call from the shop saying they want $602(!) to fix the problem. I'm concerned now because at first they couldn't detect a problem, then they found $600 worth of stuff wrong. I went and picked up the car and paid their $40 diagnostic fee, but had no work done. When I looked at the written estimate, it included the wheel hubs, but als wheel bearings. I thought it was all one piece. Is the rep[air shop trying to take me for a ride? Is this a repair I could do myself (low level of experience)?

Any help would be great!