Really depends , most 3rd party Devs are doing this outside their regular jobs
Some are all in house operations, some employ contractors to do certain parts of the modules
(FM,3D etc.) those contractors get either payed a fixed price or a commission from sales.
and like others said , there are other expenses beside time - licenses (not all have them), research (go to museums ,air shows , manufacturers etc.)
i believe an investor can get a module going for 60-80k atleast for beta stage so you can start selling.
oh and lets not forget probably paying ED for 3rd party license.