The voltage drop on a MOSFET is Vds=Rdson*I, where Rdson is a MOSFET resistance in ON state, and I is the current.
It determines the power dissipation P=Vds^2/Rdson, and power efficiency of a DC-DC converter (static power dissipation part).
In power FETs, typical Rdson resistance is in 1-100 mOhm range.
Typical currents (in DC-DC converters) is from fraction of amp to several amps.
In an inverter, one of the MOSFETs is closed, so the current is zero, and voltage drop on the open MOSFET is zero too - so that output voltage is equal to Vss or Vdd voltage.