Mars could return dividends at the same time that Luna does if they join forces.
Luna has considerable potential for manufacturing and shipping goods into orbit about Earth where it can be of financial benefit, but Luna is short of Hydrogen, Nitrogen, and Carbon outside of volatiles trapped in Permanently Shaded Regions near the poles. These elements are not only necessary for life, they are useful for many industrial processes. Mars has all of these elements in reasonably recoverable concentrations, and could export them to Luna and Earth orbiting factories.
Various groups have discussed the creation of a mining base on the south pole of the moon to mine the volatile ices found there to produce either Methalox fuel, or H2/O2 fuel to make interplanetary travel cheaper. Other resources such as water or food to supply the colony ships heading to Mars could also be produced there, saving the cost of bringing them up from Earth.
Phobos may be rich in volatiles, which are needed by Luna. Phobos could be ground up and processed at one end and the tailings dumped at the other end. It should be many years before the entire moon is converted into tailings, during that time Luna and Earth orbiting factories should be churning out a great many solar power satellites, orbital habitats, and orbiting space ports to help lift traffic from Earth. If Phobos is 30% by weight volatiles and ships six thousand tons a year that is part ammonium cyanide and part propane, then after fifty years 100 billionths of Phobos will be processed.
Initial Martian settlements will have little industrial infrastructure, and will have to import many goods such as electronics and medicine. Lunar settlements are in a good position to supply some of these needs, as the energy cost of transport from Luna is much less than from Earth.
Like Lunar settlements, Martian settlements need funding to survive. Sales of Martian resources to Lunar and other interests could be a major source of cash. The ultimate source of the cash is customers on Earth that buy power from space based solar power plant manufactured with materials mined on Luna.
To get goods from Luna to Mars, those goods could be launched from Luna with mass drivers and transferred from Earth's orbital position to Mars with a VASIMR propulsion system. Each delivery could be packed with a fiber glass composite heat shield and a woven glass fiber parachute. There would be no need to pack the parachute compactly for shipment. It would be woven in space from lunar materials. Some of the MTLMOS flights would be dedicated to launching refueled methane/oxygen retro-rockets for the final landing deceleration. Shipments to Mars would rendezvous at a space station to mate with retro-rockets while the VASIMR transport system would take a shipment to Luna.
Settlements on Mars and Luna share many needs and opportunities.
Information such as patented ideas, literature, scientific research, and computer programs can be created and traded between Luna and Mars at little to no transport expense using a radio link.
Small vessels with cryopreserved germ or eggs can be exchanged in order to enhance the genetic diversity in the population. Also, special breedings for plants and animals with an adaptation to low gravity or with radiation tolerance can be exchanged.
- Near-Infrared Spectrophotometry of Phobos and Deimos http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..156...64R&db_key=AST&data_type=HTML&format=&high=452a3fe65704274
- Phobos weighs 1*10^16 kg according to the Phobos article at Wikipedia.