1997 Chevy Lumina Strut Replacement

Suspension problem
1997 Chevy Lumina 6 cyl Wheel Drive Type unknown Automatic 105000 miles

I recently went to the mechanic regarding a noise that I was hearing a noise from my car (4-door 1997 Chevy Lumina). The mechanic took it for a drive and said it bounces a lot and the front 2 struts have to be replaced. It cost almost 400 to replace the front 2 strut cartridges. The noise continued and now he is saying the rear struts have to replaced also since they are bad. That'll be another 400. I just had 2 questions for you:
1. Are these prices the average in general (including labor)?
2. Can I go with out changing the rear struts for now, if so, how long can I go approxiametely?

Thanks you so much.