I've never had a martin house, but I do find it all fascinating.
The last house I lived in, at the back of my backyard was the backyards of the houses facing the street around the corner. The one next to mine had an old dilapidated martin house on a high pole that had belonged to some previous owners. Even in bad condition, some martins still lived in it.
New houses can cost well over $200
There is an Amish company on PA that builds and sells them.
I read that Native Americans were hanging gourds for Martins to nest in back in the 1700s.
My Aunt and Uncle next door have a big... pole? that has multiple hooks on it and they hang something that looks like different colored gourds on it. I don't "think" the gourds are real, I think they are plastic of some kind (but I could be wrong?). Unfortunately there is a big workshop between my windows and the field it is in, so I never get to see if there is activity.
I guess that makes sense where that idea came from now.
It would be interesting to know if anything besides sparrows use the 'gourds' .