I'm with Awk on this one, I wouldn't get THAT shop to do ANYTHING to my cylinder.
I have a local auto parts store chain which has a machine shop I take my blasty cylinders to. They do an excellent job, have stuff back to me in a day or three (including travel time from their local branch to the machine shop and back), and it only costs $48 for the bore job including hone and chamfer.
If that jackwagon wants $350 because he "has to get it plated", RUN RUN RUN! A few things to consider... he obviously doesn't know blasters if he thinks they're Nikasil'd, he made an egregious error quoting something before he did any homework (obviously not a very good business man), and he's too lazy to do any research into the subject. Why would you want him working on anything you have?!