Familiarise yourself with the prices.
We are mainly only concerned with the big 3

As a quick and dirty money guide:
Smokes range 5 - 25
meds range from 5 - 35
wine ranges from .7(rare) normally like 12 - 100
These are rough guesses from playing the game, there is a guide here on steam with more accurate numbers.

Reach at least the first stop on your journey, visit the gas station and buy anything that is cheap then stay in the motel. This works best at a stop where the gas station is close to the motel but it will work any at any motel.

Upon waking up, knock on all the doors in the motel. Some customers will pass you money under the door. Then get in your car and drive over to the gas station. Check to see if they are buying anything you have for a good price, and buy anything they have cheap.

DO NOT WORRY ABOUT YOUR CAR!!!

From now until you choose to stop making money you won't need to worry about it.
Try to spend every penny on items at first. as you will see your uncle will give you more soon (see how this works)

Once you've packed everything away and you are sitting in your car ready to go. Press esc then save and quit.

Let the main screen load again, open the book and press continue.

From here you just repeat Step 3 until you are happy with your money and car modifications.

When you quit and save then continue the game, it loads you back in to the game as a new day. To the extent where you can knock on all the doors in the motel again and when you first get in your car, if you have little money, your uncle will give you some! Since the shop randomises its pricing and what is prohibited every day you can just stay here quitting saving and reloading forever. Prohibited items don't matter cause your never crossing borders. so just stock up on goods, quit and save then reload and sell them for more! not buying your stuff for big money this time? oh no, just quit and save then continue and now they will ,if not they will next time. It doesn't take long and it's very easy to just quit save, continue, drive over to the store, do your stuff, then cycle over again.

Hope this helps you all get mad stacks fast and upgrade that car!
Make your uncle proud! Give me some love!