What usually gets left out of a bathroom estimate
Bathrooms are small rooms with every trade in them, so the price is driven far more by what you change than by the floor area. Keeping the toilet, sink and shower drains exactly where they are is the single biggest saving available: moving a drain means opening the floor, and on a concrete slab or a second storey that is a different job entirely. When you compare quotes, check whether each one includes waterproofing behind the tile, a new extractor fan vented outside rather than into the loft, and making good the ceiling below if the floor has to come up. A quote that lists "tile shower" without saying what goes behind the tile is not a quote you can compare.
The costs that appear after demolition are the ones to plan for. Rotten subfloor or joists around an old tub, mould behind a failed shower pan, lead or galvanised supply pipe that has to be replaced once it is disturbed, and a wall or vent stack that turns out to be in the way are all common enough that you should assume one of them will happen. Agree in writing, before work starts, how extras will be priced and that you approve them individually. Hold a contingency of roughly 15 to 20 percent of the total, and if you are down to one bathroom in the house, ask how many days it will be out of service and get that in the contract too.