Before the Industrial Revolution, clothing was extraordinarily expensive due to the sheer amount of labor involved, from prepping the fibers to spinning thread, weaving, and sewing-- when I was studying Bronze Age textiles, it was clear that it could take upwards of a year's worth of full-time work to go from raw materials to a finished set of clothes.
So, how did armies in pre-industrial societies manage to bear the high cost of producing and repairing clothes for all of their soldiers when many of those clothes would be damaged or destroyed every time there was a battle?
With great difficulty, frankly. Uniforms and accouterments were just some of the many things that made warfare in the period just prior the Industrial Revolution so expensive. The costs of war far exceeded the annual revenue-raising capacity of most states, causing them to fund wars by loans. The consequences of this are well known; both the British and French empires faced revolts due partly to war debts.
In Britain and America, some of the cost of uniforming troops was shared with regimental commanders, smaller governments, or even soldiers themselves. Colonels were ultimately responsible for clothing their men, which allowed them to contract with tailors for uniform orders (a shrewd officer might buy a clothing manufacturer, then contract with himself for his regiments uniforms). In the British army, some articles of clothing were paid for via much-hated “stoppages” - deductions from enlisted soldiers’ pay. During the American Revolution, individual states handled the clothing of their soldiers, leading to wide variations in both uniform design and quality.
It’s also important to remember the vital role repairing clothes played in the daily lives of soldiers, and the misery that accompanied absent or lacking repairs. Soldiers on both sides of the American Revolution were generally issued one wool regimental coat and two suits of linen or wool short clothes each year, though complications of manufacture and supply often meant even these modest provisions went missing or late. Soldiers had to constantly patch and repair existing clothing, often using scraps cut from clothes that had been worn to pieces. At various points, soldiers in both armies cut down tattered old regimentals into shorter jackets. Soldiers wives and children who followed behind the armies performed much (though hardly all) of this work. Still, commanders frequently remarked with horror upon the nakedness of their troops, and the insufficiency of clothing was one of the reasons heavy campaigning stopped in the winter.
Sources
Martin and Lender, *A Respectable Army *
Norton, Belonging to the Army
Urban, Fusiliers