I always simply replace the boot and repack with grease before the axle starts popping(clicking). The broken boot causes all the grease to sling out. The axle will then begin to wear out at a fast rate if you don't replace the boot soon enough. 10 bucks for the boot is a lot cheaper than 100 bucks for the whole axle. Of course, this is if you do it yourself. If you paid somebody to do it, it'd probably be just as cheap to replace the whole axle than pay him to change the boot. This is because you have to remove the axle anyway to change the boot. Get a price on the boot change and the whole axle change, then you decide.