Drainage decides everything
The first question is how waste leaves. A gravity connection to the existing soil stack, with a workable fall, is the cheapest and most reliable answer. The further you are from the stack, the harder that becomes.
Where gravity is not possible, a macerator or pumped waste system opens up locations that otherwise cannot be served, with the trade-off of a mechanical component that needs care about what goes into it.
Realistic minimum dimensions
A cloakroom with a WC and small basin can work in a surprisingly small footprint, but it needs usable door clearance and enough space in front of the WC to be comfortable, not just legal.
For an en-suite shower room, the shower enclosure size dictates the room. Squeezing to a very small enclosure to fit a room usually produces something nobody enjoys using.
Water supply, ventilation and electrics
Pipework runs are usually the easy part. Ventilation is not - an internal en-suite or cloakroom needs mechanical extraction ducted outside, and the route for that duct should be established at planning stage rather than discovered later.
Electrical work in these rooms needs to be carried out and certified appropriately, including lighting, shaver points and the extractor connection.
Where the cost sits
Broadly: drainage route, floor and wall make-good, ventilation route, certified electrics, then the sanitaryware. The sanitaryware is often the smallest line.
An under-stairs cloakroom with a nearby stack is a very different job to a top-floor en-suite over a bedroom, even though both are described as adding a WC.