In spite of post-apocalypse, dollars are still the dominant currency in the desolate lands of Michigan. They are used in trading in for items and services. Money can be obtained by salvaging pristine items from the wastelands and selling them to buyers. But hard working labor worker can earn them by simple jobs.
One known method of obtaining money is by trading at the Junk Market & ATN Enclave. Items are traded by dropping them on the floor. The current amount of money you have is above the cursor mode bar, and below the Pain Status bar.