Jeez, at a grand a pop, I'd rather just buy another gun in the different caliber.
The tube sets are $1000-1500 for a set of 3 (20, 28, .410) to fit in a set of 12 gauge barrels (usually fitted to the specific gun). If you’ve dropped $12K on a Kreighoff or Kolar, and had it custom fitted to you, the tubes are a deal.
The idea is that your gun always fits the same in each gauge, since it is the same gun. Almost all top level skeet shooters use them.