1. It's hard to give estimates due to different shops in different localities are gonna have different estimates. You'll need to call a few shops around your local area for any estimates.
2. If the cv boot is split, the best way to fix it is to replace the whole axle. I know that axles are not that expensive and are pretty easy to do. Having the boot split open allows dirt to get inside the joint causing damage in the long run. So I wouldn't recommend just the boot replacement. It's a lot more work involved too.