Drain-source resistance value depends to many factors (channel length, technology, etc.). In first approximation You have to know the channel modulation factor and calculate rds as \[r_{ds}=( \lambda ~I_D ) ^{-1}\]
Rds can be megohms to milliohms depending on the bias
conditions and device construction. Conduction losses on
the one end, leakage on the other, both matter to some.
gm*rout is gain and rout is the parallel value of internal
rDS and external load, and so on.
If you had a performance data table you could probably
find several attributes which directly depend on the
rDS under some test condition.