(This document was created in 2020 for reference use as a part of an entry in the Mars Society’s Mars City State Design Competition.)
In addressing the question of the size and value of an Earth → Mars tourism industry for the Mars City State Design Competition, there is insufficient hard data on which to draw in order to make reliable quantitative predictions. Therefore, any attempt to quantify the size and nature of a martian tourism industry will be without meaningful numerical value. As a baseline for this analysis, it will be assumed that there are one million people living on Mars and that current dollar values will be used where appropriate.
To date, the focus of space tourism industry studies has been targeted at the suborbital spaceflight tourism market. With respect to suborbital flights, the various space tourism studies have analyzed the impact of ticket price on market size in attempts to estimate total market size and potential industry revenues.
In considering the world’s current population of approximately 7.8 billion, it is estimated that 45 million people have a wealth greater than $1,000,000 US and that an additional 499 million have a wealth in the $100,000 to $1,000,000 range. 
While estimates for the total cost of a vacation on Mars are non-quantifiable at this time, Elon Musk has speculated that the price of a one-way flight from Earth to Mars may fall between $200,000 and $500,000. The singular unknown is with respect to cost estimates as to the expense of the actual stay on Mars.
Considering only the 45 million individuals with a wealth of more than $1,000,000, a recent study suggests that 18 percent or 8.1 million would consider spending more than $200,000 on a suborbital flight in today’s environment.  This would generate revenues in the range of from $1,620,000,000,000, given a $200,000 ticket price, to $4,050,000,000,000, given a $500,000 ticket price.
A key factor that invalidates the space tourism studies done to date with respect to martian tourism is that of trip duration. A vacation that includes a suborbital trip to LEO could easily be completed in 3-4 days. Similarly, using the Apollo missions as a baseline, trips to the Moon could be completed in time ranges of from less than a week to less than two weeks. 
Using the NASA Ames Research Center Trajectory Browser  configured to compute all Earth→Mars→Earth trip opportunities with maximum Delta-v set to the allowable maximum value of 20 km/sec, and set to optimize for minimum round trip time, a total of 100 round trip opportunities are identified. For these time-minimizing round trips, Mars-stay-times range from a minimum of 112 days to a maximum of 1,328 days while total trip duration ranges from a minimum of 912 days (2.5 years) to a maximum of 1824 days (5.0 years). These durations stand in stark contrast to the results of a 2012 tourism study that determined that the optimal length for a vacation is 8 days. 
The dramatic increase in total vacation duration, 152 times longer than the longest Apollo mission to the Moon, will most likely have an overwhelming impact on the number of people willing to undertake a vacation on Mars. Therefore, in terms of the Mars City State Design Competition and the size of the associated Martian economy, a martian tourism industry should not be considered as a meaningful source of income with which to cover the costs of imports.
Martian Space Tourism Citations
 Credit Suisse Research Institute Global Wealth Report 2019
 Commercial Viability Evaluation of the Suborbital Space Tourism Industry, New Space, 2018 vol 7, #2, 2019.
 The Apollo Program (1963 – 1972), https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo.html
 NASA Ames Research Center Trajectory Browser, https://trajbrowser.arc.nasa.gov/traj_browser.php
 de Bloom, J., Geurts, S.A.E. & Kompier, M.A.J. Vacation (after-) effects on employee health and well-being, and the role of vacation activities, experiences and sleep. J Happiness Stud 14, 613–633 (2013), https://link.springer.com/article/10.1007/s10902-012-9345-3