By my understanding, the X Caliber system's price is $450 for 8 adapters. The link you provided appears to be $99 for 1...MCA Sports in Alaska has been making inserts and adapters for years.
MCA Sports: Chamber Adapters, Inserts and Cast Bullets, Our Home Page!
A lot cheaper than $450
FREETHINKING like that will get you in trouble. Was waiting for someone else to add the fact that they could be home made. I wouldn't know how to rifle a barrel, but I have a lathe, plenty of round stock, and alot of imagination. Sadly, I have no old barrels, but that would be awesome..I have to tell you that with a little imagination and a couple of hours of work you can make your own 12 to 410 adapter. All it takes is a piece of schedule 80 pipe of the correct diameter, a drill and some tape. If you want it to eject the shell that will take about two hours more. Before bedtime I can make two adapters from 12ga. to 3006 with a couple of old barrels I have laying around. I could do the same thing with my 20 gauge to 45 caliber - you pick the cartridge; 45 ACP, 45 colt, 45-70. Any one of those is easily doable with hand tools, some skill, and a bit of imagination.
I've got a .45 and a .22 in pistol... looking into adding a .357 in the near future too... I'm not worried about concealed cary or self defense with this. It's likely something I'd leave at my BOL and use for hunting... not self defense...$450 buys a pretty nice pistol. And would be a backup to the long gun. Fully functional, concealed if needed, loaded, ready to shoot. Those barrel inserts are to heavy and large for practical bug out use imo. And if your using them in a bug IN situation your far better off having a complete ready to fire second firearm that somebody else can use to back you up rather than having a bunch of spare parts laying around for a single weapon.
All these same things apply to any gun conversion kits in my opinion. Their a fun idea / toy. But the negatives out weigh the positives in most practical scenarios.