replace whole axle. once the boot is torn that much the CV joint has been exposed to too much dirt and cannot be saved.
do not need to replace both sides. but inspect the other boot for cracks.
look up the price yourself at autozone.
Yeah, just inspect the other one and look for boot tears or cracking. If it looks bad, replace it too. Best to NOT buy the cheapest CV axle available because it will go bad in 4 months and you will be performing all the labor again. Don't buy a re-manufactured one either. You get what you pay for.