2002 SAAB 9-3

I have a 2002 Saab 9-3. It's been making a squeaking noise, in particular when I back up. I brought it for an oil change and the mechanic told me he thought it was my wheel bearings making the noise, but he did not have the appropriate parts to fix it.

My questions: 1) do you have a ballpark figure for how much this will cost to have fixed? And 2) I don't drive my car very often, is this a problem that needs to be fixed right away? Or can it sit for a while?

Thank you!