Retirement pay is also available. But as far as what a person can earn doing mundane tasks? IN the THB it has, under skills various pay scales. Entertainer, Professional, Prospecting, Hunting Guide, and Technical all have pay scales. Being a Broker is obviously Commissioned Based and a Professional Gambler, well.....
Most Player Characters wind up collecting pay based on starship pay charts or Mercenary pay charts (Which I thought should be in the THB or the TGP for those starting out with T20 and not having the old LBBs.).
Of all the books I have been able to gather I haven't managed to get Book 4. But doesn't the payscale there work for mercs and not for regular Imperial marines?
Look at my site (link below) in the house rules section. I have come up with sallaries for the 3/4 main careers for Traveller (Navy, Scout and Marine/Army).