This page of the guide to Dragon Ball Z: Kakarot has tips on how to earn a lot of Zeni, the in-game currency. You can spend money on, e.g. buying items or materials for your machines.
The best way to get Zeni is to get Dragon Balls and ask the dragon for gold. In this simple way, you will get 30,000 Zeni instantly. Remember that you have to wait about 20 minutes to be able to collect the Dragon Balls again and make more wishes. Dragon Balls become available after completing the Frieza Saga.
The second way is to catch fish and sell their scales. Golden Fish and their scales are especially profitable - you get 3,000 Zeni for them. Buy baits and spend some time fishing- soon you will get a lot of items for sale. Fishing can be very tedious and monotonous after a while, so you should treat fishing as a break from other activities.
Another thing you can do is to hunt dinosaurs. They are less common, but you can beat them instantly and get a lot of valuable materials. Besides meat, which is very useful in cooking, you will get their fangs, which can be sold for a lot of money.