I Do! I Do! with only that four poster bed was fairly cheap to run, even though it did have a full orchestra. Plus, why don't you count Glory Days as a Broadway show? It previewed and opened on Broadway just like any other show.
Marie: Don't be in such a hurry about that pretty little chippy in Frisco.
Tony: Eh, she's a no chip!
It is probably one of if not the cheapest musicals in history not counting cheap shows from the past that due to inflation would have cost more to produce today. I wonder how long it will be able to stick around even if the ticket sales are not great. I'd imagine their break even "nut" it pretty low.
I DO, I DO, had the salaries of Robert Preston and Mary Martin, which probably made it more expensive than many shows with casts of 10 or more at the time.
And the sets and costumes were more intricate than you realize.