Well, first off (correct me if I'm wrong) Mesa does have a five year tranferable warranty right? If you bought one that is new enough you would still have that warranty to go off of. As far as any damage these will incur, I doubt you will do too much to it, unless you are practicing for the UFC or something with it or if you are transporting it a lot without a case.
For cost you should just pay according to the specific amp. For instance if it is in absolutely mint condition, is relatively new, and comes with extras, be willing to pay a little more for it. I would shell out a maximum of probably $1400 for a Mark IV in superb condition with extras, but that is just me. I am sure to different people different things hold different values. It is all a matter of "is it worth it to me?"