A stateroom (or cabin for you peasants) will run 8,000 cr. for double occupancy. That means a cool 16,000 cr. profit. Consider that shipping cargo would only get you four grand and obviously running people os more cost effective than cargo. Wait there are a few snags. First that room runs 2,000 cr twice a month for life support each week of use so you're talking a 12,000 cr. per trip.
Second passengers are a pain in the ass. They will want entertainment, personal service, meals for GHU's sake. You're going to need a steward. `That guys gets 3,000 cr. a month. Of course a steward can tend to a number of passengers at one time. Especially if the other crew help out. You aren't going to need your gunners in jump, are you? Let them start doing laundry and tossing salad.
A person combining steward and medic is a rare jewel indeed, being able to tend to passengers' ills and allergies. The other good thing about a steward is fine cooking doesn't hurt crew morale at all! You might keep crew longer and some captains care about that.
Likewise installing facilities for entertainment and pampering can be used by the crew when they are not working a charter or are otherwise grounded.
No the ants at the picnic in this charming scenario are high passengers. For some arcane interstellar status quo fustercluck you are supposed to give these clowns a cabin all to themselves for a mere 10,000 cr.! So that'd be 8.000 cr. for that same four tons, not 14,000 cr. Worse they take up a whole ton of cargo. That's another 1,000 cr. you lose in shipping. So we're talking a bit more than half what that stateroom makes with two economical middle passengers.
I bet merchant captains loathe TAS members with every fibre of their being. Each of them get a high passage once a month? For life? Wait, we're on a regular schedule between these two worlds and this guy wants to commute regularly to see his kids/mistress/whoever?! That's 5,000 cr. we lose every month for this ... pensioner! A good captain seeks to stretch a centi-cred as far as possible. By the time he finishes you could use it as monofilament in an orbital elevator.
Okay. the price of a ticket may be fixed but any merchant captain knows several ways to skin a cat. Particularly if cat pelts are fetching good prices. Those of you who have stayed in hotels know what I'm talking about: service charges!
Basically aside from a furnished room life support, sustenance and library data access the ship is not responsible for you. Anything beyond this can be charged. Do you want to use the entertainment system? Twenty-five credits per game. Laundry service? Dry cleaning? Premium meals (i.e. anything other than meatloaf)? Headache tablets? The charges pile up. Fortunately people who can afford high passage don't mind throwing away money. TAS members are already traveling for free and might want to splurge a little.
Some merchants have elevated separating travelers from their credits to a high art,arranging shows, gambling and darn near anything else the passengers might want. What happens in jump space stays in jump space. Right?
For soe ships this is simply an economic necessity. They started out passenger liners and just don't have a lot of cargo space but they have staterooms.
Making a good name for yourself can lead to charter work. Maybe the Jionthy Atmospheric Re-Entry Club hires your ship every month to jump from the edge of atmosphere after a lavish party. Maybe you get a regular run supporting safari expeditions because you don't mind caged snarling brutes in your cargo hold (besides the trade master and deck hands). It's good economics and good andventure hooks to have regular clients to depend on who may have their own need of a chip and crew at times.
In my earlier posts I've noted that some passengers will require different atmospheres, foods, gravity levels to remain in their comfort zone. Charge them for that two. Note that stuffing rocks in the mattresses and then gouging people for zero-gee sleeping arrangements can get you in trouble fast. This is actually a viable strategy. Get enough complaints from high passengers and the Travellers' Aid Society will flag you as a flying clip joint. Then the high passengers will disappear. But then that was what you wanted in the first place, wasn't it?