My guess on the cost was not just the tickets, but on the overall cost picture. Amelia Island (the show) is in a resort destination, at the height of the tourist season. St. Johns (the show) is, frankly, just another destination in the summer in Michigan. No "high season" or anything like that. BTW, if you look in the most recent STAR issue, you will see an advertisement for a combination event that our local MBCA section is putting on at St. Johns. There will be a Mercedes class this year as well, hoping to celebrate the 125th.
Most of the shows, as you point out, have fun things. The classes you mentioned are fun. Some years ago, we had a really fun "hangar party" at Oakland Airport, with a B24, some old airmen who flew it, some Tuskegee Airmen, Bob Lutz in his Czech Fighter, Jack Roush in his P51. All kinds of fun things with the display classes, too. There were some of the character cars from the "Cars" movie for the kids. I would like to think that every year there is something new and interesting. A couple of years ago there was a "Afrika Korps" motorcycle that someone had on display, with a long trailer of EVERY TRAILER ever made for the Wehrmacht in WWII North Africa Campaign.
Personally if I never saw another Delahaye or Delage, I'd be fine. It is the more interesting vehicles in the hands of normal people (as opposed to millionaire collectors) that are attractive to me.