Actually, unless the planned bathroom space already has a window they might need to involve a roofer because a bathroom with shower requires an exhaust. Sooooo, roofing, electrical/plumbing/hvac and the permits and inspections process, drywall, carpentry, tiling and glazing, project mgmt etc and all done in a timely manner -- 50k is not bad. It may only be realistically 30k of materials and labor but the builder isn't working for free.

You also have to consider the level of finish the homeowner wants. A double vanity can be $300 from Ikea or $3000 from room and board. Tile can vary from less than a dollar for subway to $20sq ft for zellige.

A DIY kitchen for 9k doesn't really compare because no one is trying to make a profit and you're likely only making minor adjustments to existing mechanicals.

Edit: the plumbing stack will surely also need to be vented closer to this new location right? more roofwork?