1993 Maxima GL Axle Boot

I'm confused about the relationships between CV joints, axle mechanisms and axle boots-- and what to do when an axle boot goes bad. Hopefully, someone here can help me.

This is my situation. My Nissan dealer says I need a new axle boot on the front driver side for $240. I called two other places for comparisons, and was given prices of $210 - $280, but both told me their price was for replacing the boot and the axle mechanism. They do not replace the boot alone. Before I go back to the dealer to clarify what's contained in his estimate, I would like to understand:

- What is the "axle mechanism" likely to include ? That is, would it include the CV joint ? The axle itself ?
- If I'm not noticing any symptoms, would I want to pay for replacing the "axle mechanism" and not just the boot--- assuming I could get someone to do the boot alone at a price well below $240 ?
- Is a bad boot likely to be as a result of age or road accident ? (I'm concerned if age the the others could go soon, costs could mount and it might be better to trade. I've had other costly problems this year.)
- Do the quotes I received seem in-line or out-of line ?

Any help would be greatly appreciated.