Will the average person ever travel off world?
Not IMTU. My prime model for CT consists of the Dumarest books by E.C. Tubb. Most people were too poor to go off-world, or most didn't think it was an option for them. Only corporate or merchantile types, powerful galaxy-spanning organisations (e.g. the Cyclan), religious organisations (e.g. the Universal Brotherhood), military (e.g.
Mote in God's Eye), nobility (ibid., and
Winds of Gath, et al.), etc. would have the resources to travel
up.
5k for mid passage seems doable for most people if they are really interested. 1k for low passage would allow for fairly regular visits, in the same manner as overseas vacations. Perhaps once every 1-3 years or so.
On the other hand, whether or not it actually ever says so, I've always had the impression that travel in Traveller was mainly for the rich, or them that make a living from it.
It depends on what the average income may be on a given world. If we say that the typical person (the RL 'middle-class', if you will) makes about Cr2K/annum, then (if we go by LBB1p23-4) a low passage would be half what a person could make in a year. That's a substantial investment for something with a pretty risky 15% kill-rate. Middle passage at Cr8K seems awfully high, IMHO. The illusory middle-class might not exist at all on many worlds, esp. fringe worlds described in the Dumarest books which were remote, with little natural, exploitable resources, no significant industry and were only populated sparsely by the stranded poor and a smattering of idle wealthy who were there for tourism purposes.
I've read the first five Dumarest novels and there is little if any reference to money, credits, etc. but passages are mentioned with great frequency as if they themselves were the only currency. Mobility is life. Mobility is survival, and that is the key issue. If you get stuck on a dead-end world with little chance to make enough to leave, you're dead. Ships may be taking off and landing every day, but unless you have passage, you have no hope of getting off that worthless rock and you will die there. Stowaways are tossed into the airlock and flushed. Pretty bleak, but that's the main thread running from novel to novel.