High ranking priests must cost like crazy. Damn.
Simply having a mansion doesn't automatically mean it's well-staffed or adequately maintained. And being nobility is a title, in technical terms. If the land they oversee is impoverished and not generating revenue, then they're simply pauper title holders who are in the process of economically bleeding out.
As far as the actual cost and how outrageous it is...it would depend on how much value a "healthy / maintained dungeon" can generate for its surrounding region. If we're talking regularly producing big chests full of gemstones and gold and monster parts, then Resurrection might be an in-reach option for regular adventurers who have a reliable delving schedule in a maintained dungeon. Still costly, but not something you have to have a castle to afford.
But Maho's region is destitute and there's no one to set up & maintain the dungeon, so that wealth can't be generated, so the services aren't being drawn in, which causes the spiral she mentioned.