Once supply piles have been expended, generals aren't left without funding. Depending on the battlefield, the area may have oil derricks if the battle takes place in an area with oil, while closer to civilized, advanced nations there are armories. Not only that, but there are methods authorized by certain factions to create money, through legal and illegal means.
Tech structures on the battlefield may provide a secondary income for a general's operations. Once captured, they will initially give a boost of cash, followed by a constant supply of money.
Oil derricks were a common sight on the battlefield during the war against the GLA. Derricks are still used today by all sides during the Russo-European war. When captured, it will provide an $1,000 boost of cash, and a colored flag will be raised. The derrick then begins to work for the faction that took it, providing $40 every several seconds.
Each faction has their own unique method(s) of earning income beyond supply piles and oil derricks.
The People's Republic of China continues to use the Hackers that they had used during the war on the GLA and during the Zero Hour. By hacking into bank accounts, Hackers were able to divert funds into the Chinese war machine. Despite the illegitimate funding, over time they can generate more money than the other factions, about $10 per second. To protect these important agents, they are usually put into Internet Centers, which can take a lot of punishment before collapsing.
The European Continental Alliance relies on their media and public broadcasting to raise funds for their military. Unlike the nationalistic and propagandized news or biased media coverage of other nations, the ECA places restrictions on distortion of news in favor of the ECA, resulting in popularity for their honesty. ECAN based their military coverage in Telecom Relays, making both a profit and allowing the public to keep updated with military operations. In order to process and transfer data during the war against the Russians, Telecom Towers were built in order to generate funds as well as perform the aforementioned task.
The Global Liberation Army is well known for their exploit of the Black Market to finance operations. By selling outlawed items and contraband, they acquired a profit to fund operations. As well as providing money, it was also a way for the GLA to purchase important upgrades, such as AP rounds and even shoes for their slaves. By the Russo-European war, the GLA would only allow generals to build one market at a time. They can also call in a redirected enemy plane to drop in supplies at the Airstrip.
The Russian Federation still relies on its contacts in the import-export market, even after the controversy that followed the GLA scandal. Having a large number of weapons at their disposal, they regularly employ the so called Arms Suppliers to sell weapons directly on the battlefield. The Arms Suppliers are rather heavily armored but if the situation becomes too tense, up to six may be garrisoned inside an Industrial Plant for allowing them to negotiate contracts with clients in a safer place.
The United States continues to rely on supplies delivered by the United Nations, now little more than a puppet of the North American Union, to fund its war efforts. After a general has built a supply drop zone, a flare will signal a C-130 or C-17 to drop supplies, providing money to the general. In the later phases of a battle, a commander can upgrade the supply lines and earn 10% more cash when collecting supplies from any source.