Some of the cost differences are down to labour rates. I think bass strings are still largely made by hand, and some of the premium sets are almost all hand-made. So Chinese made strings will always have a lower base manufacturing cost than US made products. then marketing and distribution costs push up the price significantly, especially if it's done on a percentage basis. Most of the very cheap strings have no real marketing costs and just show up on factory eBay sites, waiting for you to find them.

Other than that it's down to the quality of materials used. A lot of Chinese steel is recycled material (especially for the cheapest products) and its quality varies a lot. So it's less likely ho have an equal mass per unit length along the string and is more likely to fatigue faster.