My mechanic replaced the sealed front wheel bearings on my 2000 Blazer at 65k miles, the cost was ~$350 per bearing with labor the total cost was $800. I think the dealer price per bearing was between $450 and 500. My bearings went bad after I replaced the tires, for a while I thought the bearing noise was caused by the new tires. My mechanic picked up on it on the next oil change. One way to tell is if the noise changes pitch as the steering wheel is turned off-center (i.e. changing lanes).
You may consider getting a second opinion from a local/independent mechanic.
Good luck,
Ed B.