How so? We have no hard data (in the OTU CT at least*) on actual passenger numbers (do we?) to arrive at what percentage of the population can afford interstellar tourism. The best we have that I can think of are the cost of living rates (noted above) to estimate general incomes on and from that most people can't afford passage at 8 or 10KCr, 1 way.
Taking that baseline we might figure out the total affluent population by extrapolation, and from that figure out how many ships are needed and how many trips are made.
* YTU, MTU, and other rules may have added data or changed baselines