No, it’s not necessarily expensive. A ZWave radiator thermostat is ~60-80€, and you can get a MAX! for as little as 30€.
Speaking of motorized or thermoactive valves to control underfloor heating, I got them for 12€ per valve, and you need just one actuator per zone, not per loop.
Now since the valves are usually located as a cluster in just one or two rooms, you will be able to use multi-actuators such as a Pi with a multi-relais hat, or any other device to have enough connectors (I’m using a 6-socket ZWave plug for ~90€).
If you still think that is expensive, go ask your local HVAC dealer for a solution.
Any single device (to contain electronics) they offer will be 100+ € … that’s probably what most people still have in mind.
EDIT: Now that’s my German (or European) view on things. Housing installations can vary greatly throughout the world, though, see e.g. @rlkoshak’s post on U.S. housing below, and so does the effort of any attempt to change it.