to clarify, you are actually just getting the torn boots replaced, not the CV axles. the parts costs are relatively minimal, i.e. less than a $100.00. however, it's a somewhat labor intensive process for a DIYer w/o a lift. in short, $400.00 to replace all your boots, if I read correctly, is a very good price.
finally, I would not drive w/torn boots. once you start hearing noise from the axle, I think you've likely hit the point of no return, so I wouldn't rely upon that. the purpose of the boot is to keep debris, i.e. dirt, from entering into the axle.
-jimmy
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