What did Joshua say rebuilding Jericho would cost the builder?

Answer: His firstborn and youngest son

Joshua swore that whoever rebuilt Jericho would lay its foundations at the cost of his firstborn and set its gates at the cost of his youngest.

“At that time Joshua invoked this solemn oath: “Cursed before the LORD is the man who rises up and rebuilds this city, Jericho; at the cost of his firstborn he will lay its foundations; at the cost of his youngest he will set up its gates.””— Joshua 6:26 (BSB)