The TPWS uses existing sensors (ABS) to compare wheel rotational speeds, so no extra hardware cost, just a bit of coding.
The TPMS does, as you point out, use relatively expensive sensors. These typically last 5yrs and as the vast majority of new cars are lease/fleet buys, this won't be an issue for them.
Second hand buyers will have to face the replacement costs though it is feasible to replace the batteries at a fraction of the cost. A member here has done just that.