You won't find those deals on ebay, it's craigslist or FB marketplace. There's been a bunch of them up here lately, at least 5 sets have gone up for sale all $300-$400. I bought one of the Mitutoyo 0-6 six mic sets. The 6-12 inch sets are also springing up around the same price, though they have one 12" frame and the anvils for the smaller sizes.
That said, I doubt you are turning cannons right. It could be a couple of mics is all you need for gunsmithing, maybe 0-1 and 1-2. If that is the case and it were me, I'd go super high accuracy Mitutoyo electronic mics. I have 3 of them covering 0 to 3 inches. Anything I do over 3 inches in diameter won't need more than .001 accuracy.