what are the symptoms of a leaking axle boot? Is it expensive to repair?
The best way to detect a damaged axle boot is visually, when the car is up on a lift for an oil change. This is just one of the many reasons to have a real mechanic, rather than someone at a quick lube place, servicing the car.
If an axle boot has been split/damaged for an extended period of time, dirt and grit will enter, thereby causing a lot of damage to the CV joint. By the time that you notice the symptoms–such as a lot of clicking when making turns–it is too late to do anything other than replacing the axles.
So, if you hear clicking noises from the front end when making turns, you need to have this investigated before you become stranded, as the joint(s) can fail at any time once you hear these noises. On the other hand, you might be able to drive for months in that condition, but it is very unpredictable.
Preventing a problem is much cheaper than curing one, so be sure to have your axle boots inspected each time that the oil is changed. If caught promptly, replacing just the axle boots can save you expenses for parts, as compared to having to replace the entire axle. However, the labor costs are essentially similar.