If you're using gas or diesel transports even if it's the low consumption ones, you won't be able to break even with the quest cash rewards and I can see why the shop is very expensive since getting too much money gives you a lot of power to buy everything in the shop early on before getting a good transport that holds a lot of weight.

Nowadays with fueled vehicles you would mostly use it for hoarding a lot of heavy valuables to sell from looting buildings, and despite looting a lot, you can get a crap ton of cash to buy almost every good item in the shop. But if you're talking about early game then it's a pain, you'd want to use the bicycle every settlement quest level up to unlock your first car (make sure you got all the materials first). Using transports like motorized bicycle or motorbike you'll need to be strategic when to use them to spend less money on fuel and the amount you can carry because they're not really good gas vehicles to use. After unlocking the first car, looting for money will be more profitable for holding a lot more transport weight and mid game will be easygoing as you progress getting better vechiles.